A subtle difference between this and cross-platform play.
Sony has announced that cross-platform play is coming to PlayStation 4, starting with Fortnite. But cross-platform play wasnt the real reason people were upset about how Sony was handling Fortnite accounts. The anger came from the launch of Fortnite on the Nintendo Switch during E3 this year, when players found out that any account that had been played on the PlayStation 4 couldnt be used on the Nintendo Switch.
Source: N4G PC PS4-locked Fortnite accounts now freed for Switch and Xbox One, merged accounts coming