Mm, something that I was thinking of. Sabi's source has deemed the leak as "unlikely", but it took them like 3 whole days to do so (maybe a bit less, but not that much less). One possibility that comes to mind as to why it would take so long to deconfirm the leak is that insiders know that either Geno or Doomguy are coming next, so the sudden appearence of the Mii suits has confused them. If they're expecting Geno and suddenly see a Mallow Mii suit they would be like "oh wow okay, that makes sense, is that leak true?", while if they know the next character is, to say a random one, Rebecca Chambers from Resident Evil, and the Mii suit leaked was a Mallow one they would be like "okay I know that's wrong because it doesn't make sense with what my sources told me". The fact that some insiders were a bit thrown off because of the leak, even if the leak ends up being false, might mean more than we think.
Anyway, until someone shows how someone faked this I won't completely disregard the leak. It's still so much work, way more than the Grinch leak, that it's still hard to believe some could fake it.