Most of my points have more or less been made in this forum, but I'd like to summarise it again:
1. Nintendo didn't say that they won't (pay attention to the grammatical tense) make the controller compatible with any other software other than SSBWiiU, but that it isn't compatible with anything but smash 4, hence the quote "The GameCube Controller Adapter for Wii U is compatible with Wii U and Super Smash Bros. for Wii U specifically - it is not compatible with any other Wii U software"
2. The vWii (Wii mode) on Wii U didn't originally have the ability to play Wii games utilising the Wii U gamepad but now it is possible to use it, not only as a screen, but also a sensor bar. This seems very hard to code to get both features in compared to USB controller support, but Nintendo still did it for their consumers due to demand (although it isn't exactly what they asked for). This gives us the possibility of GCN support for vWii.
3. New Super Mario Bros. U originally didn't have support for the Wii U Pro controller, but it was later added in a later patch. Mario Kart 8, or any other popular multiplayer game could have future support for Gamecube controller via a future patch.
Of course, this doesn't prove anything 100% as an argument could be that the Gamecube controller isn't as known as the Wii U Pro controller, or that's it is different to the Wii U Pro controller which literally has the name Wii U, while the Gamecube controller is a 2 generation old controller.
You could also say that the Wii U Gamepad couldn't be used as a controller for Wii games, but you can't forget the fact that the Wii U gamepad has an identical control scheme to the classical controller minus the extra features (touch screen, clickable triggers a.k.a L3 and R3)
With 8 Player smash in mind, I always thought that the GCN controller wouldn't be used as a Wii U Pro controller because the Pro controller has a significantly different layout scheme and Nintendo are very strict with that, as you can't even use Wiimote (and nunchuck combination) for some games, with Super Mario 64 being an example.