No, Game Pass Ultimate cloud gaming for Xbox games is on an “Xbox” (I’m sure MS has some custom servers with blades of Xbox-like servers). PC games, yes, they are doing something similar but it’s not quite the same as GFN.
GFN allowed you (not sure nowadays) to just login to your steam account on a RemoteFX Remote Desktop and play games via their client. This setup is what the tool is replicating. RDP allows some crazy tweaks in the registry to let you get really good performance out of supported hardware and (I assume) is what is going on here.
Edit: this is Linux based it seems so probably just doing something similar as the MS RDP implementation.