I think they’re using Widevine DRM. And with DRM they can enforce whatever arbitrary policies they like. They set special restrictions for Linux. I think Amazon set 480p as max, Netflix 720p and YouTube 4k or sth like that. AFAIK it has little to do with technology. It’s just a number that the specific company sets in their configuration.
Thanks for spreading the word. We get these complaints every few weeks. More people need to be educated and move away from these instances to make the Threadiverse a better place.