I am unsure if this is just a terrible oversight by QA, or something Sony mandated; but apparently, if you log into Fortnite on PS4 at all, you will then be unable to use your account on Switch.
That seems to be an issue with Fortnite itself. I don't see how Sony could stop anyone from logging on Switch on a game that belongs to a third party. At best it's Nintendo that should be blamed for not letting people on THEIR platform log on a game for whatever reason.
But I don't even think that's the case, it must be a purely Fortnite issue because I assume the account you can't log on is a fortnite account and not a Sony nor Nintendo account.