WebKit core gets commits from nearly every major hardware manufacturer and OS vendor that isn't Microsoft. Unless Apple's stripping all those commits out of spite, iOS WebKit contains code from Nokia, RIM, Samsung, HPalm, Google, and many others that might be seen as competitors otherwise.
Adobe is more or less neutral, though (they just want you to buy their tools). Apple, Nokia, RIM, Samsung, etc. all would very much like the others to cease existing; yet they're all contributing to a project that keeps each other on a level playing field (in the context of HTML/JS engines, anyway).
I think the point is that Apple didn't treat Safari/Webkit as being a competitive advantage so much as nullifying a competitive disadvantage -- it was a defensive play, not an offensive play. (A nice analogy is Android, which Google developed out of fear they would miss out on the mobile market rather than out of any desire to "own" the mobile market. Google didn't want Apple or anyone else to own its destiny, just as Apple didn't want Microsoft to own its destiny.)
