I think you got a bit confused… FEX is used in the steam frame (VR headset) because it uses an ARM processor to save battery. The steam machine uses a normal x64 CPU and appears to be using some relatively standard pc hardware, so no compatibility layers are needed for windows (only drivers are needed)
I doubt you’ll be able to install windows on the steam frame though, for the reason you say (arm compatibility is a mess).
I think you got a bit confused… FEX is used in the steam frame (VR headset) because it uses an ARM processor to save battery. The steam machine uses a normal x64 CPU and appears to be using some relatively standard pc hardware, so no compatibility layers are needed for windows (only drivers are needed) I doubt you’ll be able to install windows on the steam frame though, for the reason you say (arm compatibility is a mess).