That is a backronym[1], it absolutely meant X Desktop Group and likely changed to "Cross Desktop Group" when they switched to Wayland. D-Bus, .desktop files, MPRIS are all listed as FDO specifications alongside the Desktop basedir spec and none of them are appropriate for macOS either.
FDO applies to Linux and "Unix-Like" Operating Systems. macOS is not "UNIX-like", Apple still bothers to get it certified under UNIX 2003 so it is technically not a Unix-like. Again, just because it has a /usr folder and a /var folder and can run a bash shell out of the box doesn't mean all the same mostly just OK standards from Linux should be copy-pasted over.
[1] https://lwn.net/2000/0427/a/freedesktop.html