Bottom line is that Valve doesn't want to pay for Windows licences, that is all.

I thought the all this Valve on Linux work came about back when it looked like Microsoft was going to really push their Windows Store.

So less avoiding Windows licences and more avoinding getting cut out by MS.

Both, which is kind of ironic, as they need to emulate Windows and DirectX, and have been a complete failure making studios port their PlayStation, Android and Switch games to SteamOS, despite the heavy POSIX flavour of those platforms, specially in what concerns Android and its relationship to Linux.