Valve was using Debian way-back-when, but the pace of getting new stuff into debian proper is too glacial for Valve. Valve is putting a lot of work into “making the linux graphics stack rather good for games”, and having those improvements integrated upstream quicker means that Valve can get to work on the next set of improvements.

Valve is still using Debian as the basis for their runtime environments for games (pressure vessel). Debian’s slowness is great for providing a stable ABI for the parts that come into contact with (seldom maintained) game code. There is some amount of magic that goes into gluing the stable runtimes with rapidly changing stuff like Mesa.