Why would Sony refuse to allow diablo 4, or call of duty for that matter, to appear on the PlayStation? The most contentious thing about the entire activision deal is the titles that currently release on PlayStation becoming exclusive to Microsoft platforms, there’s no chance Sony would ever block those games from their platform arbitrarily just silly business move.
In the future, these games could come with their own nested game stores, bypassing the platform store. It wouldn't be an issue for Microsoft because they own both games and platform. It might be tough to accept for Sony.
WHo knows? Times change, leadership changes, priorities change. I never thought Sony, a Japaanese company, would abandon their Japanese branch of first party development, but here we are. It's simply Gran Turismo and some support wings to help out 3rd party developers. Oh, and a singular VR team that hasn't yet announced its first time since the scale down.
They were able to promise all CoDs would be on PlayStation, so presumably the same way.