From my understanding Apple effectively owns your entire output when you're employed full-time (at least as an engineer). At a recent interview I mentioned I like tinkering with code on the side and was told I'd have to give that up and stop contributing to open source. I imagine that extends to participation in the "outside world" at least to some extent.

The fact that Apple developers I've spoken to spend their personal coding time (on the rare occasion it exists) on work projects only reinforces all of this.

(All that said, my sample size is seven people, so I could be quite wrong in the end.)