most of them are windows only and dont run on linux, i smell windows kernel inside for windows titles only . steam might purchased windows kernel and injected it as emulator inside their O.S.
It's Linux. They've already said as much. About 250 games available today, which is 10x more than XBone or PS4 (and 10x less than Steam on Windows).
Remember, Windows PC on same network = can play Windows games on SteamOS via streaming