He built his own version of the user interface framework, the software that sits right above the graphics processor on an iOS device. Apple creates that for developers — it was completely unnecessary for him to do this. But this is the kind of thing he considers “fun.”
It’s reading stuff like that that makes me either (A) want to throw away all my Objective-C programming books and give up or (B) hit the books harder and learn faster. I’m still not sure which it is this time, though.