Honestly i'm on team fake for this one.
Just looking at the screenshots, you can tell right away that some things aren't exactly right.
- The Green shell looks like it was simply plasted on Mario's hand.
- The models look very similar to the 3DS models. (They have no detail on them and they have a cel shaded esque look to some of them.)
- All of it just looks fake, seriously it looks like fan art someone has created or fancy photoshop rather then actual game engine, and honestly I can tell right away that this is a hoax because it looks a bit obvious in some places. (At least that's how I see it personally.)
But who knows, With E3 coming next week,
This could very much be real,
The only thing we can do is simply wait.