If that was the case they would be on Origin and would be bringing at least their back catalog to GOG too (EDIT: Or, you know, building their own storefront with their battle.net team!). They're on Steam because it it the healthiest environment for AAA $60 shooters on PC. In any case, Activision have no reason to throw their weight behind a platform that, by design, will arbitrarily cut them off from Windows 7/8 customers who have been playing PC COD for years. Literally the only reason they could be there is either for a payout or some kind of contractual obligation.
Microsoft are not providing "healthy competition" on this platform, they are distorting it and causing confusion to end users by introducing another PC SKU for any title that supports both UWP and Win32. These SKUs have different OS requirements and now can't even offer a guarantee of either playing with other Xbox users on console, or other PC platform users.