Earlier quoted context omitted.
Apple's already done what you're asking for. The Darwin kernel is already open-source, released mostly under the Apple Public Source license. It's the kernel for OS X and iOS. Granted, there's not much userland...
But it's not open source in the way Swift is. Apple doesn't have a git repo or something for Darwin. In essence, you only get the "tags". They don't take pull requests, you need a Dev account to post bug reports, etc.
It feels a bit dangerous to claim that the only True Open Source projects are the ones hosted on Github and Github alone. Just because the source code isn't released using your favourite version control system doesn't mean it's not "open-source." The code is available and yours to use.
I agree that Apple's darwin release doesn't have much of an open-source community around it.