I meant the 19c and 21c are not necessarily same. What is referred to as "pmemdb" in 21, to my best knowledge won't be available in 19c. Also, the code for "pmemdb" is there for 21c, and so is documentation, but some last minute finishing touches for a v1 that went into 23 didn't make it to the cutoff for 21c. 19c will have fsdax, yes. REDO logs, can be (to my knowledge, and I am just one developer) can be created on an XFS (DAX) fs. pmemdb distinguishes itself from this by allowing the buffer cache to allow byte reads directly from datafiles, as long as pmemfs and data file block sizes match.
Admittedly this is indeed a bit confusing, so the keyword is "pmemdb". The pmemdb link applies to 21c (or 21.3 or 23, when they appear), not to 19c.