This bit of info was unearthed during Miyamoto’s latest trip to Paris where he was interviewed by various media outlets, one of these outlets was El Mundo — a Spanish newspaper — whose entire interview can be read here (Spanish).
Aside from this reveal not much else was shared by Miyamoto regarding the game. He did, however, admit that more and more traditional games are becoming available for iOS and Android devices; in response to this, they’re creating new types of games.
Aside from the Wii U Mario game, Miyamoto is also heavily involved in the development of Pikmin 3 and Luigi’s Mansion 2.
Could this Wii U Mario game be related to Nintendo’s claiming of the Super Mario 4 web domain? It most certainly seems like it. I guess we’ll just have to wait for E3 for official details.