I'm not saying Epic is a bad poster child for this case because Epic is a sleazy company (although from all accounts they kind of are); I'm saying they're a bad poster child because their battle is running up a steeper grade.
Are they arguing that Apple's policies make it impossible for them to split money with authors at the same rate on ebooks? No. How about the policies making it impossible for them to give 100% of event ticket sales to their users and impossible for them to even tell users they'll make less money when people buy tickets on iOS? Nope. Maybe they make it impossible for them to put a client for a streaming game service, period, full-stop, even if Apple got a cut of the money? Negatory.
What does Apple's policies make it impossible for Epic to do, then? Get a higher cut of the revenue from Fortnite's game currency when it's bought on iOS. That's what got their developer account terminated. This is the stand they're taking, the flag they have planted, the hill they are ready to die on: that 70% gross profit on zero marginal cost virtual tchotchkes isn't enough.[1]
Maybe this case will be successful, but my suspicion is that if Apple changes their position, it's going to be either due to regulatory pressure or the ever-increasing weight of the rolling PR disaster they're getting themselves into.
[1] Edited to add: As I wrote, I think it's ultimately about them wanting Apple to be forced to allow the Epic Games Store to be installed on iOS through the Apple App Store, but the case they're bringing is still about Fortnite.