Here are the messages you get when you try to log into an Xbox or a Switch, with and account that logged into PS4.
"This Fornite account is associated with a platform which does not allow it to operate on Xbox. Neither the Fortnite website or Epic Customer Service are able to change this. To Play Fortnite on Xbox, please create a new account."
"This Fornite account is associated with a platform which does not allow it to operate on Switch. Neither the Fortnite website or Epic Customer Service are able to change this. To Play Fortnite on Switch, please create a new account."
The general consensus across the entire internet is that it is Sony's fault. The only time the issue ever comes up is if someone tries using an account that has logged into a PS4 at some point. Never when dealing with any other system, PC, phone, ect.
Even if it's Sony's fault, Sony has NO POWER WHATSOEVER on what happens on a Switch or an Xbox so at best Sony asked Nintendo and Microsoft to block Fortnite accounts created on PS4 and both Nintendo and Microsoft are then accomplices to this. So worst case scenario all 3 brands are just as guilty, though Sony could be the one asking for this cause we all know Sony does not want to go for cross-platform gaming.
Saying that Sony blocks an account on Switch for Fortnite sounds as ludicrous as saying that Sony blocks Forza on Xbox should Forza not work on Xbox for some reason. I could conceive this if Fortnite was a Sony IP, but it's not so again only way to get this blocking to work is with the complicity of both Microsoft and Nintendo... And also the owners of the Fortnite IP must be accomplices.
The other way around would make more sense: Someone creating an account on Switch for fortnite would see it not work on Sony cause Sony can block anything it wants on Sony hardware, obviously but Whatever machine something is blocked on is THAT machine's fault primarily.