I must have missed when Valve open-sourced their game engines and started pressuring the same as a preference for games on Steam.
Oh wait, they haven't. Why should Valve give a shit about open-source drivers? If it's cheaper or easier or better for them to push NVidia/AMD to open their drivers in order to spur quality-parity with Microsoft Windows, they'll do that. If it's easier for Valve to just pay NVidia/AMD to improve their proprietary Linux drivers, they'll do that. I suspect they'll go for the latter, unless NVidia/AMD have some deep collusionary hijinks going on with Microsoft, in which case Valve will reluctantly push the issue, assuming they can. It's even quite likely that they just won't have that clout, and would shutter their Linux initiative rather than get into a brawl.
Linux is just a strategy for NVidia and, frankly, a precarious one. They'd rather not have the Windows Troll around, but it wouldn't be an easy battle, and NVidia has many other avenues for staying in the game. Either way, It's not about ideology for them, so arguments based on that are silly.