Obviously a lot of work would have gone into this if it's fake, but it's still less work than faking Geno or Doom Slayer as playable characters while achieving the same result
This my main hang up.
Think of every single detail about a playable character. All of their animations, sound effects, lighting, shadows, character portrait, name, etc. If you wanted to fool people with a video leak of a new character, you have to get ALL of this 100% right. You can't have anything that looks or feels off, and people will examine every single frame and be quick to find any details that aren't right.
In contrast, most of this is already done with a Mii Costume. The animations are already done, the sound is already done, the lighting is done, the shadows are done, the name is done. Heck, even the portrait is done in a way because how Mii fighter characters portraits work. And for a Mii Costume that's just a hat, 2/3 of the character model is also done. There's way less things that can go wrong and let people on to the fact that the leak is a fake.