Back in those days, Apple was all about open-source - I still have an OSX 10.2 (Jaguar) box somewhere where the main headline is about Apple and open-source.
While there might be less shouting about open source contributions nowadays, arguably the contributions themselves are much more useful.
Certainly WebKit has been widely adopted outside Apple, clang/llvm is slowly, but surely gaining converts, and libdispatch is IMHO deserving of wider interest, even though it may not be widely adopted on non-Apple platforms.
I think they were just trying to piggy-back on BSD reputation as stable and mature system, so they played the buddy-buddy card. Once OS X settled in its place, this was no longer needed, so the open source love faded.
(Just found this: http://www.flickr.com/photos/stevenf/5393674266/)
I guess there's still a lot of open-source in OSX, but they don't need to shout about it now that they are no longer the underdogs.