My personal guess is that they wanted to release the printed spec before they released the code. The spec has now been released, the 10.14 source has not, maybe apfs.kext will be in the 10.14 source drop.
(also, HFS+ and its fsck have always been open-source with OS X)
HFS+ was first released for Mac OS 8.1 in 1998. The earliest source code I see is from Mac OS X 10.0, which was released in March of 2001 (not sure when the source was released). The first iPod was released in October of 2001 which used HFS+. I imagine by the time that code was released it had been battle hardened quite a bit.
Because on two separate occasions, APFS ate all of my free space resulting into me reinstalling macOS as the system was locking up. (Yes, I filed a radar, and yes they fixed it, only to regress 6-8 months later).
A global lock on readdir().