Quote from Mr. Aonuma himself:
“The game will actually be released in 2018. It will have a overworld the size of Twilight Princess, but will be more innovative because you won't have to steer Epona; it was this top-notch mechanic which took us roughly two years to perfect. We hope you like...
http://www.nintendolife.com/news/2015/10/the_legend_of_zelda_on_wii_u_and_30th_anniversary_on_the_agenda_for_upcoming_famitsu_feature
Here is another article covering the topic. Apparently they never really said it was going to be about Zelda Wii U, just The Legend of Zelda and the Wii U for...