I wouldn't come close to saying it's a lack of matchup experience, Fumo has been playing for long enough where I wouldn't be surprised if he was familiar with every matchup.
I'm also not trying to pass off 6B/throw as some godlike mixup. It certainly is not garbage but it's not something you just throw out often. It's like saying TRM is garbage, it still gets the best of us. You can't 100% count on reacting to the 6B every single time, there are times where you just can't predict with certainty that it's coming, thus why it gets blocked ever. Yes, blocking beats it. Yes, jump outs can beat it. Yes, reacting beats it. Sounds like virtually every mixup in the game. It's a decent mixup with great rewards when it works, and it's not terribly high risk (you're in the air thus removing some risk, not to mention most people who try to mash out are literally mashing the A button). While it may be just a footnote, all footnotes wind up being supplementary.
EDIT: You all need to remember that this discussion arose over the mixup directly being called a "gimmick", and I disputed. I never once said it was exceptional, nor it was his best.
Arakune's 5A is 6 frames. That's average. Not a good counter-argument. Folding under pressure is human error.