I get what you're saying but people are way too quick to throw characters into high tier just because a player picks them up and does well. Keep in mind his claims that they're top 15 means he's saying they're potentially better than characters like Mario, Cpt. Falcon, Inkling, Pichu, Falco, Roy, Chrom, Bowser, ROB, etc.
Even if she's top 15 many of the characters you mentioned could be top 10 or higher up on top 15, like Mario, Inkling, Roy, and Chrom. I do think she's better than Captain Falcon, Pichu, Falco, Bowser, and ROB though, but it remains to be seen, I suppose.
I don't see myself being optimistic until more than 1-3 players do well with them. I want to at least give it time before making a true final decision.
This situation reminds me a bit of Smash 4 Corrin and Lucina. Many people considered Lucina to be much, much worse than Marth, often putting her 10 or more spots lower than Marth. This was, of course, false, and Lucina was either slightly better or slightly worse than Marth (I'd guess slightly worse at top level, slightly better at high and mid level). Still, many people didn't change their mind about Lucina until ZeRo started playing her as a secondary and Mr. E as a main. Corrin was also underestimated for a long time by the Smashboards community, many putting her in high-mid tier, citing a lack of results. I claimed that when Cosmos would finish school and start traveling people would see what Corrin could do, and well, I was right. Having played her myself and followed the Corrin meta I already knew what she could do and that she had the tools to be high tier, but most people didn't follow Corrin and mostly just viewed majors and such, so they didn't realize how good she truly was until Cosmos started taking names.
Corrin, in this game, has long been a low-mid tier (since 4.0.0, before 4.0.0 she might've been a low tier, her recovery was terrible back then). While she had some players, most notable players moved away from Corrin, which makes sense: Corrin mains generally don't play her because of her personality or because they're fans of her origin game, they play her because she's fun to play and good. Well, low-mid tier Corrin wasn't terrible to play, but she felt... a bit off. Like her numbers were bad, and a number buff would make her good again. Guess what 8.0.0 did? It gave her kill power. It gave her damage. It gave her more safety on aerial pin and f-smash. Corrin, after 8.0.0, is much, much better, and she's almost definitely the most buffed character since Ultimate's release. She might even be the most buffed character from patches in any Smash game, ever (it's either her or Smash 4 Mewtwo).
Does that mean that she's top 15? Wellllll... I don't think she is, personally. Right now, there's no consensus amongst Corrin players: Some think she's top 15 (like Cosmos and SHADIC), some think she's top 20, some top 25, some top 30, and some even top 35. I've noticed that players coming back to Corrin from Smash 4 tend to be a bit more optimistic than players who picked her up in Ultimate (SHADIC being a notable exception), and that solo mains tend to be more optimistic than those that play her as a secondary or together with multiple other characters or don't really use her at all in tournaments. Just an observation. Do I think top 20 is set in stone? No, it's plausible that she's "just" top 25, which is still high tier and viable anyway. I do think she has what it takes to be a top 20 character, though.
I also think she's about as strong or stronger than in 4. The most significant buffs, compared to 4, are her recovery (which is noticeably better, and this also buffs her edgeguarding), her kill power (much stronger), f-smash being much better (both the chainsaw and as a raw move, now being more safe on shield and killing earlier than in 4), pin cancel existing and pin jump being much safer, and her combo game and kill setups (in 4 those were 50/50, now they're true). Oh, and universal ultimate changes, like reduction of landing lag, also benefit her, although perhaps not quite as much as they benefitted Lucina (Lucina is probably
slightly better, but not 10 spots much better). The most significant nerfs from 4 are pin kick safety (it travels less far and has more endlag), though at least pin cancel and pin jump are options. And, honestly, if you watch Cosmos or play her yourself you'll notice that she can still get away with pinning unsafely. It's a mixup how you react to pin on shield, and if you guess wrong she could just pin kick you. Unsafe pins are less safe than in 4, but still pretty good, and the reward is better (earlier kills).
Oh, Dragon Fang Shot is a mixed bag. The projectile is much better off-stage, now occasionally
killing people. The stun doesn't last as long though, so uncharged shot usually won't lead to optimal bite, though you could either charge the stun slightly or just bite earlier. It takes a while to get used to if you're used to Smash 4 Dragon Fang Shot but overall it's a mixed bag, some buffs and some nerfs. It's still a good move and now you can edgeguard people off-stage with it more effectively.
Her counter is flat out nerfed. I don't think it's a huge deal, since it was kind of overrated in 4 anyway (most good Corrins didn't counter that often), but it's definitely worse now. It probably hurts the most versus Pikachu, since she can no longer just counter Thunder Jolt spam (in 4, it had 1 frame of vulnerability, making it incredibly hard to punish; in Ultimate it's more like a regular counter and much riskier versus projectiles, it also kills much, much later, which isn't a huge deal but makes it worse. I don't think Counter Surge was an important part of her kit in 4 but it's still a move that got strictly worse (I guess the counterattack window is 2 frames longer but that barely matters).
So... Corrin is mostly considered worse because one move got a bit less safe? While she got a ton of other buffs? Yeah... The more I think about it the more I think Corrin is actually better than in 4. Dash attack is also usable now, the reduced endlag means she can sometimes get a fair out of dash attack making it much better, and f-tilt kills which is nice as an emergency kill option. Up-smash is a bit better, down-smash is less laggy (the tipper backwards kills a bit later but 5 frames less endlag is nice and it still kills pretty early). While fair is slower (9 instead of 7 frames) it's probably overall better since it deals more damage and combos better than in 4. Corrin gets a lot out of short hop falling fair.
Note that even if Corrin is better than in 4 that doesn't necessarily mean her tier list position will be relatively higher. The meta has changed, she now has to face potentially losing matchups like Joker and Snake that didn't exist in 4, and Palutena got buffed and might be losing (though some put her in even), and Pikachu got buffed and is definitely losing (though I think it's only slightly losing). Greninja is generally considered to be losing in Ultimate as well, although some think Greninja is even. Fox and Zero Suit Samus were losing in 4 and probably still are. Aside from them, I don't think she really has any losing MUs, but time will tell. That's seven slightly losing MUs, which fits well with my idea that she's top 20.
Some matchups that I currently think are Even but could easily be losing: Captain Falcon (losing in 4, got easier in Ultimate but could be losing I guess), Diddy Kong (losing in 4, got easier), Inkling (Cosmos thinks it's losing, most other Corrins seem to think it's Even from what I've seen but Cosmos used to be an Inkling main so perhaps he's right about Inkling, might be hard to estimate since he's both the best Inkling and the best Corrin, maybe if he plays Space or someone...), Roy (I think it's even but Roy is pretty beastly so could be losing), Sheik (losing in 4, easier now but could still be losing, really hard to juggle her), Shulk (winning in 4, but he got a lot better). So I guess that's... Between 4 and 13 slightly losing MUs? Fox, Joker, Pikachu, and Zero Suit Samus almost definitely losing (I think -1), Greninja, Palutena, and Snake probably losing, Captain Falcon, Diddy Kong, Inkling, Roy, Sheik, Shulk possibly losing but I'm leaning Even.
Not everyone will agree with this and some people will list other characters as losing and winning. Right now there's no real consensus overall. SHADIC put Ryu and Mega Man in losing, Cosmos put them in winning, for instance.
Notable matchups that she might win? Bowser, Ike, Mario, Luigi, Mega Man, Min Min, Mr. Game & Watch, Ness, Pac-Man, Olimar, ROB, Rosalina, Young Link, potentially Peach, potentially Pokémon Trainer, potentially Wario (I have those three as Even right now though). Sonix the Sonic main has struggled vs Corrin online but I'm not sure if we can tag it as winning for Corrin, though it doesn't seem to be impossible for Corrin to win the MU, she has very good tools vs Sonic. For now I peg it as Even.
So... Yeah, I think Corrin is very good, potentially better than in 4. If people think she's worse (as a character, not in the meta) than in 4 I'd like to hear some arguments why that is the case. Based on what I know right now, I think she's likely top 20.