Yeah, Amiibo Costumes aren't confirmed, but there's a whole lot of stuff pointing to it.Amiibo Costumes aren't deconfirmed, and 100 Mario Challenge is basically the story mode.
From what we know, the big mushroom probably isn't exclusive to the 30th anniversary 8-Bit Mario amiibo anymore, so it's likely a replacement for the amiibo mushroom. And the amiibo costume probably wouldn't work with the multiplayer.
Also, uh, how is 100 Mario Challenge like the Story Mode? It's likely replaced with the endless mode, which is good, except for the fact that you only start with 5 lives, which is a huge problem if you like Super Expert.