This actually stems pretty far back to rumors that were somewhat blown off at the time, but now knowing what we know, this rumor may have been very accurate afterall.
http://www.nintengen...k-gpu-info.html
Basically, we know that the Wii U doesnt use Direct X 11, thats a microsoft API, but it was assumed that OpenGL would be the API in its place. Well, that code may very well run on Wii U, but may be far less efficient that Nintendo's latest API, rumored to be called GX2. You can read up on the original GX api for gamecube and wii here:
http://devkitpro.org/wiki/libogc/GX
GX shares some similarities with OpenGL, and differs greatly in many ways as well. OpenGL, by design, masks a lot of the nitty-gritty hardware specifics, leaving implementation of it to hardware vendors, whereas the GX API is very close to the metal and many functions have little, if any, processing performed by the CPU. What this means is that, if you write smart code and know how the hardware works under the hood, you can bring out the best performance of the machine, but you'll also be working with an API that is altogether more complex than writing in a higher-level API such as OpenGL.