In almost all cases of standards you can implement those standards without reading the document, from an IP standpoint. From a practical standpoint it is often just not feasible to reverse-engineer everything without the original documentation, or worth it if you can't slap the trademarked name of the technology on it.

It may be possible that AMD could even implement an open source driver stack for HDMI and be legally in the clear. What they fear is more souring the relationship and losing access, so they don't risk it when they were told not to do that.