Valve have paid developers to work on the AMD Mesa drivers for years at this point, to improve Linux as a whole and ensure Linux drivers are in a good state for upcoming and future AMD hardware. Valve invest in a lot of different areas for Linux, not all of it specifically just for their own hardware. A lot of their contracted developers just work on regular upgrades and improvements like this. Valve have done this for a long time now that we've been covering here.

Standout example: Valve made ACO as a better-performing alternative to LLVM for compiling shaders; it was sufficiently better that Mesa also switched to using ACO rather than LLVM. Just standard open source ecosystem improvement things.