Haha please. Epic developed the game. They ported it. It runs on their servers. They would know the account restriction policy being imposed.
It's their customers. Like I said, they are as much at fault, not that they are solely so, and should have warned their constumers about their own game.
I'm not sure I'd lay as much blame on EG as this. Yes it's their game but it's not always their network through which accounts are created and linked to other networks. I downloaded Fortnite and created my EG account through my Switch. I imagine EG has limited control on what the customer gets to see during this process. I also expect that to use Microsoft's and Sony's respective networks, EG has to play by their rules...
Sony has been the one to push against cross-play, there currently exists no cross-play between PS4 and Switch/X1 (X1 and NSW are able to cross-play and play the same accounts fine from what I understand), and the one EG points the finger to when trying to play a PS4-linked EG account through the Switch is PS4. If there's one place where I would expect Sony to have less control over what the user can and cannot see during EG account creation and such, it would be non-Sony platforms.
This is why I lean on Sony as being the most responsible for how things have turned out.