Here's two other things that have me suspicious enough to the point of calling this fake.
First off, why did the leaker show us the screen and the GameCube controller, but not at the same time? If this was the real deal and the leaker really wanted to convince people, then he should have shown us him pressing the buttons on the controller as Spring Man jumped and attacked. This would be EXTREMELY hard to do if this was just pre-recorded footage, and people would have a hard time discrediting this if he did. However, it seems like the leaker deliberately moved the focus of the camera from the controller to the screen before Spring Man started moving.
Second, Spring Man stays in one spot and does a fairly simple attack. Trying to animate a character model to move around, jump, across the stage, and attack with unique animations all while making it look official and smooth is pretty hard. Trying to animate a character model to jump up and down? Ehh... Not so much. Again, if this was the real deal, then I don't see why the leaker wouldn't try to show off more. It doesn't have to be a bunch of unique specials or a Final Smash or anything. Just seeing Spring Man run around would be helpful in validating this. Instead, the leaker showed us something so basic and simple that just about anyone with a little experience could've animated.
None of this is conclusive evidence against the leak being real, but I feel like this should be a little brow-raising for some people.