The letter is only in regards to the Unreal Engine being blocked. It will directly impact Microsoft and other game developers not directly involved in this fight.
They did not mention Fortnite or alternate stores at all.
That's 100% at Epic's fault.
First of all, they willingly put their customers at risk.
Second of all, they could've put the engine business in a different entity, keeping it safe from their games business
They did put the engine business into a separate entity, a Swiss company called Epic Games International S.à r.l. Apple is threatening to cut off access for both entities.
That’s Apple’s prerogative after banning a developer who has violated their contract with them. Otherwise you could have shady developers just create numerous of legal entities to effectively brute force their way on to the store, and Apple would be unable to respond.
This case is less about monopolisation and more about inequality between two parties when settling a contract. Nothing stops Epic from making an Epic phone and doing the same.
They did not mention Fortnite or alternate stores at all.