The comparison to GeForce Now is wild, because that's a service running "your" games on someone else's hardware. Of course Sony can walk up to NVIDIA and say "fuck off, we don't authorize you to run God of War for your users" and NVIDIA will say ok we won't run God of War anymore.
Steam distributes game installers. Sony can't make Valve produce a special version of Steam for the Xbox that tells its store that it's on an Xbox, especially if the Xbox in question is literally a windows PC with a UI skin and a lock on administrator access. I can run Steam right now on my android phone via winlator and download+install a Steam game.
What Sony could do is patch restrictive DRM software into all their games that checks the hardware info and fails to boot if you're on "an xbox".