Some people on Era are now saying its actually like this:The latter seems to be roughly true.
XSX = 1024 GiB physical (1100 GB) = 920 GiB available for system = 802 GiB free for players (OS = 118 GiB)
PS5 = 768 GiB physical (825 GB) = 690 GiB available for system = 664 GiB free for players (OS = 26 GiB)
Not sure which of this is true.
The former I don't think is. But it's hard to say. Manufacturers very often will use GB to mean GiB.