If I were Valve, I would absolutely require isolating Steam players from the Microsoft Store buyers. The last thing Valve needs is leaking its customer base to Microsoft.
When a game has multiplayer servers, is Valve involved at all?
The steam API allows servers to verify a player owns the game and provides a server list. Actual networking once players are connected isn't part of its scope iirc.
Don't know, but it doesn't really matter. Player segregation would be a clause in a contract between Activision and Valve first and foremost. How it's technically implemented is secondary.
Probably, a big factor could be the anticheat stuff - without the steam client inspecting game state, Microsoft store players could get away with aim bots or other exploits