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.
Always interesting (and good for a laugh) to see the mental gymnastics people come up with to try and blame Nintendo for someone else's policies.