The sources seem to have it right when you look at the official game page that says the eShop download requires an External Hard Drive. This leads us to believe that it isn’t able to fit on the typical 32GB Deluxe model of the Wii U.
Besides the eShop version, there is also a retail version, but it isn’t clear how much space the retail version will need; but we suspect that it clearly will need to download at least some. Still, it is a small price to pay for the game, which we have to admit looks pretty darned good.
You can read some more here.