There is a lot about the Geno situation we do not know, but I can say, due to the amount of people who has indeed heard Geno, I find it really hard to believe it's "leakbait", I also wouldn't pass anything off as "leakbait" anyways because time and time again this year, the idea of "leakbait" has been disproven by the 3DAS collection, Steve and Sephiroth, 3D World port, etc.
I can't count how many people has heard of a Mallow Mii, so it's good to hold out hope when it comes to Geno, this costume probably doesn't deconfirm him in my opinion and feel free to harass me due to that.
I see the Mii as another obstacle currently, not a hard deconfirmation, and Mii costumes can already become playable characters, so the rule has already been broken.
What could of happened if Geno gets in as a fighter, they didn't want to make whole new contracts with SE when it comes to the costumes, so they just decided to bring the Geno Mii back and then make one of the biggest reveal comebacks we've seen.