That's what doesn't add up to me. if that was the case, why even bother to stand up a Windows 10 infrastructure if those are already available as the Xbox version. It's a different license, sure, but if MS has the pull with third party publishers to make those PC licenses streamable, then why not just stick with the Xbox based infrastructure? The list of PC-only games currently available on the Windows store seems far to meager to make this worthwhile. Now, if it included the other storefronts, that's an entirely different ballgame...
Well, I the last article I read about pc streaming and XCloud, it was saying access to the "3000 Xbox one games as well as Windows released games on PC". I get that it's not everything, but it's a lot.
It remains to be seen what types of games/publisher support Stadia/Nvidia Go end up getting, tbh.
Maybe Microsoft is offering up their azure based xcloud for major publishers to use for their own services? Maybe that's why they are removing themselves from other cloud streaming service providers and how you will get such games technically through XCloud?