The bolded part is wrong. An account that has been logged into on PS4 will not work on the Switch. Even if you made your account on PC or mobile and never actually played a game on the PS4 version and only logged in to get one of those two free skins they gave away to PS+ members. Those skins being a major contributor to the anger here. Sony literally incentivized people to log in on their platform.. now seemingly to deliberately block them from playing on two of the other four offered platforms... without saying a damn thing about the ramifications of it.
It is slimy, it is despicable, and it is costing Sony a bit of rep and cash right now. As it should.
Yes, since they were the ones against cross-play, this blowing up in their face is only fair. My point was that EG is also to blame... But let's move on as that part has been discussed at length.
Personally what seems the most wrong from Sony is that (correct me if I'm wrong) they did not put a warning. A message just before you logged on Fortnite with the PS4, something that says: If you log on your PS4 with this game it will not work on other platforms, or something. Note that EG could have had that warning too since you must log on their account to play the game on their servers.