Honestly, while the last couple weeks have proved that Nintendo is relatively bad at managing final reveals, the longer this mystery goes on the more it’d honestly feel insane(in a bad way, of course) if Geno just turns out to be a Spirit.
Nearly three years ago, Geno’s Mii Costume got its own trailer in the middle of the big final Smash 4 direct, and a couple months later, Sakurai had that now-famous interview.
Clearly, if Geno is just a Spirit, the correct procedure should have been to directly show off his fight for a few seconds during the final Direct, complete with his costume, since precedence indicates that he has quite a notable fanbase surrounding him.
The incorrect procedure would be to briefly show him as part of a screenshot on a menu option, linked to one of the only characters in the roster who is literally known for seeing the future. And then completely hide his existing costume in all possible appearances, while making it unclear if that menu appearance even deconfirms him as a character.
I think it’s worth reiterating the point about the menu thing, actually: We have not seen Geno’s icon specifically chosen for any of the lobby footage. We have just seen the menu option with him on the screenshot, but there has been zero recorded Ultimate footage so far where someone deliberately chose Geno anything, or saw the option to do so.