But the question is not whether it's legal to reverse engineer a binary. The question is whether someone with no affiliation to neither the HDMI forum nor AMD could contribute the according code to the AMD driver which is part of the Linux Kernel, regarding legal implications.

Reverse engineering of binaries is pretty much a solved problem from the legal standpoint.

It's hypothetical. There is no difference between those two scenarios, we could also just assume the hacker pulled it out of their ass...