On one hand... Weird how Rayman, another third party, would get in over other Nintendo characters. One piece of proof that makes me lean towards this is that Sakurai stated himself that they provided him with a full 3D model for Rayman that was specifically made for Smash. If they could provide Sakurai with that, I'm sure it wouldn't be too much work actually putting him in, as it would be to make a character from scratch.
The one thing turning me off from this leak is that the guy went through the trouble of uploading a video showing him in the roster, but why not go one step further and show Rayman in combat? Like the guy with the Bowser Jr. and Shulk videos. Also if they're moving characters around, why not put Lucina, Doctor, and Dark Pit with their respective series? =/
I remember greatly enjoying the Rayman games for the PS1. I don't recall playing any after that... Maybe I will someday. I hear great things about the more recent games.