Just decided to do post-posting research and it does run a modified NT kernel, but still.

So where does Xbox make money? It’s not the hardware. It’s the games. Seems like they should stop developing a “store” and a “platform” and just adopt Steam. I don’t think it runs any version of windows underneath. Refocus efforts in improving Proton, develop great games, offer them on Steam. My Steamdeck was the best mobile experience I ever had for some really great games that also worked on windows. If I could install SteamOS on my Series X, that would be a killer Steam Machine.