Apple continues to open source their version of XNU, although there has been an increasing lag between the release of each OS version and the corresponding sources. The latest available sources are from 10.11.2, three minor versions behind.
Apple has also shifted to pushing a lot of sensitive/proprietary code into kernel extensions (the new Apple File System being one example), for which they don't release source code (generally speaking).
Filesystems, by and large, are supposed to be pushed into the kernel (via extension, or direct compilation). I'd hardly say that they've "shifted" into pushing code into their kernel, but that much of what differentiates Apple's XNU kernel vs FreeBSD's Kernel or Linux is what they choose the exclude.
I believe the grandparent post is referring to the fact that HFS+ is available in the open source release of xnu [1] while the question of whether Apple will open source APFS kext, especially given their recent trend of moving functionality from xnu into closed source kexts.
I think it would be smart for them to open source it, if not simply for the interoperability use cases.
Yep. So far, they've committed to publishing "the APFS volume format" [1]. It'll be interesting if they fold it back (into the kernel proper) as part of making APFS bootable.
Apple has also shifted to pushing a lot of sensitive/proprietary code into kernel extensions (the new Apple File System being one example), for which they don't release source code (generally speaking).