It's a bit strange that Sakurai revealed this stage considering he wanted to avoid spoilers and this basically confirms Wario (although to be fair the "W" symbol in the April Direct did that too, and who really thought he would be cut anyway?)
On the topic of that leak, I gotta say I'm a bit skeptical, Sakurai said he didn't want to make it seem required to buy both versions, and limiting characters to Wii-U transfer would do just that for 3DS owners. (I guess it could just be DLC, which would be fine)