You are correct.
But KH3 would have sold better on Wii U than on Xbox One. Xbox One owners won't pick up that game, I bet my apartment on it. There is no reason for Square Enix to release KH3 on Xbox One and skip Wii U.
JRPGs with mainstream hype will sell enough on Xbox to justify a port. Also Xbox one will have around 20-30m consoles sold by the time KH3 comes out, too much for SE to ignore for a mainstream appeal game like Kingdom Hearts.