Not sure what the new rules for Xbox are regarding such games compared to the old rigid 360 "penalty fees for updates". Given how "online everything" these new consoles have struggled to be, it should be far more flexible now.
However, I don't think that's the reason that a cross-play model has not been implemented. I'm not sure what the reason is though, because I thought it would be inclusive. Perhaps the p2p netcode is the reason thay Xbox players cannot be share PC/Mac seesions? Because they will use a different online system that Microsoft provides for the Xbox.
It makes the whole online nature of this game even more pointless if the entire player base cannot interact. Yet having said that, the 32 player limit also makes it no as big a deal anyway, since we PC/Mac players can't simultansously play with all of our own playerbase anyway, and as such this is not an MMO like everyone keeps misunderstaning.
I understand the P2P nature of the game. I did wonder if one of the reasons they were coming to Xbox One first was because MS may have offered Azure in replacement to the currently used Amazon Servers, but who knows?