I bet Blizzard is and has always been violently opposed to putting Diablo on Gamepass. (Probably have no love for their new overlords either). I'm thinking Phil's recent visitation to Blizzard was to calm the waters by giving them about a half a year until it's ported over. They can technically get around not having it on their because it's way too late to claim "Day One" on Gamepass.
Darth Phil, starts to motion towards the door, stops, turns around and exclaims "Pray, I don't alter the deal any further."