IIRC, a non-AMD contributor was working on adding HDMI 2.1 support to Linux's AMD driver, and then AMD managed to get the HDMI Forum to see that their position was moot and was making things very awkward for AMD.

Oh this makes sense! Obviously, it can't be illegal for a non-forum-member to add HDMI support; it's just an HDMI forum policy after all, not law. That's certainly the link I was missing for this to make sense.