Why would the licenses be incompatible? AMD owns the code, so they can release it under whatever license they want, including GPL or BSD. They could even put into the public domain if they wanted. AFAICT, the code has not been released at all, so this point about licenses is simply wrong: it doesn't have a license at all right now.

As for a broken driver, that's an easy fix I think. From my reading of the article, there's already an existing driver, but it doesn't support HDMI 2.1 features. So it's simple: provide a fall-back driver, and require users to separately download the new driver (or maybe distros can package it themselves, I'm not sure about the legality). If something goes wrong with the unsupported driver, leave an option in the boot menu to boot in a "safe mode" that uses the old driver. So they won't get 4K @ 120Hz, but I'm sure they can live with that.