You're confusing Trademark with Copyright. The Open Group owns the Unix trademark, it does not own the API. The fact that POSIX uses the API does not mean any claims to "structure, sequence, organization" of the APIs or command line utilities are invalid.

This has never been litigated before. Past UNIX copyright wars were fought over copying actual code between Unix and clones. No one has ever tested whether API reimplementation of Unix is copyrighted. Implementing as public spec won't save you. Java has a public, open, specification and was clean-room reimplemented in much the same way that Linux is not Unix, Android Java is not Java.

Claiming there is 0 danger in being sued sounds like naively wishful thinking, especially with Apple's deep pockets, there are plenty of lawyers out there looking for ways to crack that nut.

The Open Group own both the UNIX trademark and the POSIX spec. They used to demand a lot of money for copies of the spec, but you can get it online for free nowadays ( https://www2.opengroup.org/ogsys/catalog/G101 is a good place to start). They put copyright notices on the spec, but (at least until this ruling) that was widely believed to only apply to the description of what functions needed to do, and not to the function declarations.

Is it clear that The Open Group or IEEE actually have a clear right to claim copyright of the POSIX api given this ruling? Were they assigned or licensed that copyright by AT&T or any of its successors? It seems likely enough that in the 80s this was not seen as necessary, and the old guard of UNIX implementers were still being quite belligerent towards each other at the time, afaik.

I believe the Open Group was assigned whatever copyright AT&T had when the Open Group was created.