MS can't do that though. Apple said the devs have to. So for every game on Xcloud each and every dev needs a dev account and submit the app/game themselves. Right now Xcloud has IAP disabled outside of Samsung devices. Money on it that Apple tries to make it so they have to turn IAP back on, they get their cut but MS doesn't.
Devs can do that easily. Being an Apple developer isn't hard. MS could just create a tutorial for the smaller developers who might need help. The "apps" are just streaming wrappers anyway. There would be a bit of setup and organization for all the XCloud games at first but its the easy part of these new rules.
Apple hasn't forced any other developer to turn on IAP back on (like Netflix), don't see why it would be different here. IAP will just probably not be available.
But this is assuming MS does actually think this is worth it, which they might not.