I also believe the use of the term APFS format documentation is just that – an expression they have used instead of referring to an official title.
The Apple File System Reference is almost certainly what they refer to. This reference was published on September 17, 2018, and Alsoft made this statement on October 4, 2018.
Prior to the Apple File System Reference, there was no consolidated documentation from Apple regarding APFS – at least not to this level of detail. In fact, the most comprehensive documentation of this was actually the result of private academic work in the computer science area.
As for Apple itself, we have had the following documentation:
Apple File System The reference was last updated in February 201