I think many of the built-in are actually pretty out of date, but it does seem like Apple's not too concerned with following the upstream release schedule - one of the Xcode releases shipped with a slightly pre-3.0 release of LLVM's clang.
Apple has been responsible for a lot of recent LLVM development, so it's not as though they didn't know what they were getting into with that.
Everyone ships with the development 4.3.x series, because stable Zsh releases are very rare and the development series is actually very stable. It is not just OS X by far.