There is a lot of confusion because of the 5GHz radio used to communicate with the Gamepad. However that is handled by a different Broadcom chip. Ifixit did a teardown of the Wii U and the WiFi chip is BCM4336 which is 2.4GHz only. It is also pretty obvious from the specs as the Wii U is advertised as BGN which is nearly always 2.4GHz only.