... heck programmers have a hard enough time jumping back into their own code they wrote if they haven't looked at it in a while (good commenting doesn't always help on a big project like this)!
Good commenting always helps. The factor you are ignoring is that of "not so good" comments that are perpetrated as 'good commenting' doesn't help. All those "insanely great" comments written in a 2:00 a.m. frenzy of caffeine haze really aren't as good as the author thought they were. Or the carefully and mechanically written to appear "good" that matches the code's algorithms from 6 version control check-ins ago. Same thing goes for the source code. It is far more mangled and lacking essential insightful variables names and structure that it is pitched as.
The problem for most of these software projects is that this never enough time to do it right, but always time to do it over (or fix it later) .
Same thing for this kind of borrowing from OS X. It happens every couple of releases. ( "Yeah, we'll fix the resource allocation problems later..... just get this one shipped. " )