The Doomguy disconfirmation means nothing to any content but Doomguy playable in that context.
Sans was licensed more than a half a year from its actual costume debut. And it's pretty likely to get just a Mii costume, actually. It's a character mascot that's an easy costume for them to make. Nice simple cash. It's not remotely weird to get a costume for just the mascot(this applies to Sans too, as he's not the player character. And what several costumes? It's literally one, just the Cacodemon).
That's also not how leaks work. Leaks almost never tell all the details(...Yugioh lists aside) when it comes to video games. They're piece meals at best. With small bits. Even Smash 4 had multiple small leaks that told us details, including ones that became outdated due to changes in the main plan. That's why Chrom and a Rhythm Heaven character(it's not clear if it was Chorus Kids, but probably was. Just the actual information we have is only a leaker saying the name, and we have a playable icon only for the franchise, which doesn't lead to much) were on the list. Sakurai only didn't mention the RH character planned, but did mention Chrom planned. More importantly, he confirmed that it was a "Pokemon from X/Y" just like the Gematsu leak said. The most he might've made up or guessed is which RH character it was at best.
Here's the thing; the fact the costumes haven't shown up yet? Of course they could be fake. But it not showing up yet isn't saying much. If they got made recently as is before showing, they were just as likely not ready by Byleth. That means only the character after that was possible. ...It's literally 6 months between Byleth and ARMS. That's the reason why it's such a wait. Nothing else. Why wouldn't you wait if your only option was later on. Honestly, I wouldn't be surprised if they were proof of concept and couldn't be used because the IP owners said no. That's also a distinct possibility. Maybe that's why they didn't come with Byleth either. They may never show up again if they were only proof of concept to begin with. Development's not just real or fake, as it can get more complicated.
Do note I'm leaning on fake due to the video not being taken down. It's reasonable evidence of it being fake, but the rest of the stuff are based more on observations. Since we know a costume can actually be in development for well over 3 months straight before being released, we know the time period doesn't honestly mean much at all. Cause, well, again, if it wasn't ready by Byleth, it was going to take a lonnnnnnnnng time. In addition to that, it could've taken a while to license as is with SE and ID Software. These things take time. Or there's just too much time between a character release to begin with, so unfortunately one leak lingers on due to that alone. It's also clear it can be faked(but nobody has come out with their own video, so that doesn't mean much. They can't actually recreate the process either, so we may never know for a long time, even if it never shows up). On the other hand, it may never show up ever even if it's proof of concept. Sakurai has to wait a while, and he can't really say things went poorly that easily. I mean, gotta keep on the company's good sides.
It's a weird situation.