I guess AMD is just as bad as Intel is for host bus (HBA) support.
Plugging in my SATA port multiplier into my X570 as expected was not recognized. X570 evidently is not compliant with the SATA standard.
I install a cheap Marvell SATA card, connect the SATA port multiplier and it works. So this tells me AMD/Intel are taking steps to prevent people from installing more hard disks.
Marvell's literature on their logic shows full support. So this means AMD is not compliant.