I thought they mostly open-sourced the GPU driver for Linux? I interpret it as a way to compete against(proprietary) CUDA dominance, but maybe that is wrong?

https://github.com/torvalds/linux/tree/master/drivers/gpu/dr...

Yeah, and their main reason for some proprietary drivers existing is usually 3rd-party proprietary dependencies. E.g. they open sourced AMDVLK by starting with the closed Vulkan driver and replacing the proprietary compiler dependency with LLVM.

Much of the AGESA code for booting the system remains hidden.