What? AMD develop their open source drivers and submit them to the kernel and they have their own set of Linux specific closed source drivers.
Back then it was basically just Intel's integrated graphics that had first-class mainline support. The AMD linux support is a relatively recent shift historically speaking.
Intel really should get a lot more recognition for its mainline Linux graphics support though. Without their contributions over past decades we might not even have a modern graphics stack at all in-kernel.
https://gpuopen.com/amd-isa-documentation/ for AMD.
https://www.intel.com/content/www/us/en/docs/graphics-for-li... for Intel
ffmpeg is one very small but important part of a complex chain.
Or, I guess the other option is that you’re spreading misinformation.