I'm still baffled that 3rd parties give the platform holders the right to do this with their account systems, especially locking the account to their platforms in the case of Fortnite.
When a Playstation user plays with a PC user, it is on the PC server ran by/for the 3rd party.
For Sony it makes sense, Switch largely isn't getting Final Fantasy 14 because of this reason for instance. Cross play also nulls the advantage of walled gardens like XBlive and PSN, if your friends can play on their platforms of choice and you can still play with them, it doesn't force you to buy their platform of choice, so again this is more about Sony holding their position, which in the short term makes a lot of sense, but with a new generation starting, the more savvy people, (lots of early adopters) might be looking at this as a deciding factor.