It's definitely Windows. You do see the Windows 10 chrome on windows (sometimes the steam is there minized) and games in steam install to Program files...
Oh for sure they're using Windows to host most/all games, with a very good justification for doing so, but the way the client app interacts with the server they could just as easily have some games hosted in Linux. The point being that they are in no way tied to Windows, it's just the best option for what they're doing.