Modders didn't say it wasn't possible to do these costumes. They said it was incredibly difficult. And there's a world of difference between the two statements: the world of fake leaks.
Indeed, it's way easier to think it's a real leak, because of all the work it would imply from the person (or group of person) who did it, but so was the Grinch leak. It's easy to say that the Grinch leak was "obviously a fake" nowadays, but, let's be honest: it wasn't that easy.
The purpose of this new leak, whether true or false, is not to "show things in advance" but to initiate discussions on the very nature of this leak. And probably torment a bit the community.
The bare minimum to make a fake leak real is to make the community thinking it can't be a fake. Finding out how to mod Mii costumes, and create some totally new, is the bare minimum. It takes a huge amount of work, but some dedicated guy could do it. Notice that it's "only" a hat, and not a full costume or a whole fighter.
If it's a legit one, it doesn't fit with the usual Mii costume announcements related to a fighter... unless the said fighter isn't one. If these Mii costumes are released alongside Terry, either Geno and Doomguy aren't fighter themselves... or they are also released alongside Terry. And that's also a farfetched assumption. Or these costumes are made way in advance and that build of the game is only available to only a few, probably the NA QA team.
After what we lived with the Grinch leak, I'm really leading toward a fake.