Great question.
The CoreFoundation ("CF") that ships on Darwin is a superset of the CF that is part of swift-corelibs-foundation. It includes things like cfprefsd, which (as you noted) are not part of our open source distribution. However, nearly all of the core implementation is the same.
We have stopped shipping tarballs to the opensource.apple.com site and prefer to drop updates to CF on GitHub instead. We believe this is a better model for maintaining it going forward. We are striving to keep them in sync at a cadence matching at least the major releases of macOS and friends.