Epic has their own servers and their own data. From what I remember about the cross-platform scandal it was sony that didn't want to have online interactions with other platforms it couldn't regulate. this is 100% #EpicGreed behind a facade.

Cross-platform play != Cross platform sign in.

So long as Sony is excluded from matchmaking in cross-play mode it shouldn't be an issue. So again I point to #EpicGreed and say how is this even legal? You are the ones who have the data, you are the ones who have the server. Sony has no business telling you how to access it. So I know for a fact its all just a get-rich-quick scheme. 

Stop being lazy and properly implement a platform lockout without having to lock out the accounts. Its literally all you have to do to meet the demands of Sony.