Epic said "Okay, we agree.", then broke the terms they agreed to, so Apple is closing their account.
This seems like pretty straightforward break of the terms of use to me.
The reasonable way Epic could have proceeded is to have submitted an update with the "third party store" stuff added, gotten it rejected, and then rolled it back while suing Apple.
Instead, Epic pulled a publicity stunt by sneaking it in, specifically so that Apple and Google would remove their app, and then suing, in an attempt to make themselves look like the good guy (which, depending on your perspective, might be true).
It doesn't feel any more mature than goading someone into throwing a punch at you so you can claim to be "defending yourself" when really you were just being a douche and you got what you asked for.