I'm not convinced that the ROG Ally is designed as well as the Steam Deck, and I'm less than impressed with Asus in recent years, but I do love open systems. This seems like a good move for Steam OS.
I think this was their plan all along. Maybe it wasn't, in any case I was sure it wouldn't be long until we see SteamOS (or something like Bazzite, which is already here I don't know how functional it is) on these things. Why you would advertise Windows on a handheld PC I didn't get at all. The certain titles you are only able to play on Windows (bc poor support or anti-cheat etc etc) usually aren't games you would play on a mobile device anyway.