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.
[1]: http://opensource.apple.com//source/xnu/xnu-2050.18.24/bsd/h...