Wii U owners will be able to download a standalone version of the challenge mode. Though that mode apparently may become available on the other versions, the Wii U version has an exclusive mode that requires the Gamepad. The other types of play will be available on PS3/360 when the game launches in September, but the Wii U will host an exclusive mode of play within Challenge mode.
Consider this: why would they release something in April that required something that comes out in September? That doesn't make any sense, right?
As the video explains, this version is downloadable exclusively from the eShop starting in April; PS3 and 360 owners have to wait until the game comes out to try it. This is also taking the place of the originally-planned second demo, meaning that you do not need the actual game to access it. It is completely standalone.