Players being held back by bad character choices is certainly a thing, but that's really only a legitimate concern for like, I dunno, Kirby and Villager mains and the like. Certainly not for Wario mains, whose character is solidly in the top 20-30% of the cast and has been for the game's entire competitive lifespan.
Completely disagree, you can be held back by anywhere from a few to many spots of achievement regardless of whom you main. A common sentiment during early Ultimate was that Nairo was being held back by Palutena. Leo expressed this a few times, saying that he thought Nairo could be a contender for #1 if he mained someone better.
Every player is carried by their character, so it goes without saying that Gluto would do worse if he piloted a weaker character or didn't play Wario, but every player is held back by their character's limitations as well, and Wario's limitations I think are especially brutal.
Overall, I actually think Wario is pretty mediocre. His hard matchups include zoners/campers like Pacman, Olimar, Sonic, Steve, FGC/Kazuya, and ZSS, Pikachu, Cloud, Roy, Palutena, potentially Mario, and more besides. Wario's matchups look so... mid tier or below to me, holistically. He has strengths against sword characters, so you could convince me that he would have developed even matchups vs. Cloud and Lucina and winning matchups vs. Byleth and Corrin, and I'm certain that he obliterates large-bodied characters, but that is a still lot of losing matchups for an upper-tier character. It's like three entire archetypes that he struggles against.
d-tilt whiff punishes and full drift in nairs aren't as good as Gluto makes them seem; this character doesn't have the neutral tools for a favorable matchup spread against the good characters
You might as well say Sisqui and Yaura are being held back by playing Samus because the Olimar matchup exists.
Sure, that is just vacuously true, but Sisqui and Yaura also lose consistently to other players, so their results would improve slightly -- less worth noting. Yaura also does play Cloud.
And frankly, if maining Wario truly is holding Gluto back from even greater accomplishments than what he's managed to do up to this point, then yes, it is an indictment of his overall skill level if he doesn't recognize this and pick a character that better allows him to win. The competition begins at the character select screen, after all.
This is just another way of saying that acola and sparg0 have the best results -- which is obviously true; there's no point in discussing whether or not they're the best players by results because, yes, they are. No interesting discussion can be had with that as a starting point.
Every player who isn't #1 is held back by something -- some missing skill set or circumstance. Identifying that Gluto in particular is held back by Wario's losing matchups instead of some deficiency in his neutral or lack of polish in his combo game or lack of ability to adapt isn't hollow rhetoric, it's an observation of his unique position in the meta.
When top players play friendlies with Gluto, they frequently walk away with the impression that he's the best player at the venue; Gluto's h2hs vs. other top players are frequently in 2nd or 3rd place, probably behind only acola. Gluto has beaten two of the game's all-time greatest players in more than 5 sets in a row.
These are qualities that are worth observing and noting.
Also, as an aside, I hate the maxim "competition begins at the character select screen." True competition begins with passion, not a robotic decision to optimize win rate.