I have seen mention of the deep password linkages from multiple sources, but from personal experience I believe that it is of relatively recent origin — like High Sierra recent. Given what I know about APFS and how it relates to the rest of MacOS I believe any tie in there to be a very remote possibility. Any password linkage in the storage system would be in FileVault, not HFS+ or APFS. The underlying file system is isolated from and theoretically invisible to the rest of MacOS. (This isolation of functionality is at the heart of the Unix concept not to mention modern programming technique.)
As far as a KB article on the subject goes since Google, DuckDuckGo,
et. al. can't find anything, I think you are going to have to dig into Apple's
Develpoper's Documentation probably under
Security.
NOTE: the Developer's Documentation assumes a working knowledge of Objective-C and Unix so be forewarned.
