A lot of thoughts [1] have been put into the usr merge, and compatibility with unix is one of them, and one other unix that has done this merge is Solaris, so Linux distros doing the merge are not even that special.

(note: it's not about merging /bin and /sbin)

[1] https://systemd.io/THE_CASE_FOR_THE_USR_MERGE/

If "being UNIX" hinges on the fucking filesystem structure I honestly am fine with it "not being UNIX"

It does, however it is actually less strict than I had in mind,

https://pubs.opengroup.org/onlinepubs/9699919799/

I suspect you missed the sarcasm in Athas' comment?