They're right - if Apple enabled Android to use iMessage, then Apple would take a good hit to their user base. Walk up to an iphone user and ask them the best part about their phone - the majority I've talked to, say the very first thing is iMessage.
It's not the iPhone that's keeping them, it's not a particularly stellar phone - it's iMessage, and their perceived struggle it would take to switch off of their iSuite. Android phones have a significantly richer experience and Apple knows this, and the fact that if they open up one of their services to the competition, it will most definitely hurt them.
That being said - it's well within Apple's right to keep services internal to their ecosystem if they so desire, it's their platform. Pretty weak argument by Epic here.