I really can't see the argument here. Apple developed their app store from scratch with their money, and linked it to their inventions, the iPhone & iPad developed with their own money, and continue to run the app store using their money to support software and hardware. Yes, they now make a lot of money out of it, but that was the intention of it in the first place. It's their app store, and they set the terms and conditions of people/companies who use it. If you don't agree with the T&Cs, don't use it, no-one is forcing you to use it.
Epic, and everyone else, is free to setup their own, competing, app store, and create their own devices to access these apps. Epic is just looking for a shortcut to an extra few $100s of $millions. I hope they lose this case.