I have read that too from the topic that was linked in the Devolution thread. Basically, just because something works doesn't mean it works well. :P I myself was under the impression that, even before he had made such a comment, that it worked but was buggy. Considering he hasn't uploaded another video of him actually PLAYING a game on the Wii (rather than just sitting at the title screen), it's my guess that it still has major functionality issues - perhaps it requires a convoluted process just to connect the controllers currently?
Some people have also speculated that he may be "holding back" until he can implement some sort of working anti-piracy method on the Wii U, but I personally think that's silly since we could at least enjoy the extra controller functionality on a normal Wii right now without any other changes - it's not like it'd break Wii U compatibility when it's not usable right now anyway.