Almost nothing Valve has worked on is only for SteamOS, other than packaging and distributing SteamOS itself. They've upstreamed kernel patches, RADV patches, KDE patches, etc which affects all desktops. Not to mention the open source tools like Gamescope and Fossilize, the latter of which is used automatically on all Linux PCs playing Steam games, and their contributions and funding to Wine and other projects. Even the new Steam big picture UI, which was initially only available on SteamOS, is now broadly available.