Between the ZFS-like features and the advertised "novel copy-on-write metadata" scheme, I would not be surprised if it was partially based on DragonFly's HAMMER/HAMMER2 filesystem.
If you poke around in the the APFS kernel extension, it's not a very big binary, given the feature set. (It's a 550K extension, compared to the 2.5MB zfs.ko on FreeBSD.) I haven't disassembled it yet, but I'd wager that APFS is layered atop Core Storage btrees/CoW. Since that code's been shipping since 2011, maybe APFS stabilizes faster than a true greenfield filesystem?
(That does leave me wondering how interesting an open-source APFS would be without an open-source Core Storage.)
