To sell a game outside Apple’s App Store, developers must effectively pay a 50 euro cent per user per year installation fee once they reach a certain number of downloads. If developers want to link users to purchases outside the app, they’ll also need to fork out a 10 percent commission on all sales made “on any platform” — including outside of iOS. That’s on top of a 5 percent commission on purchases made within one year of the app’s installation. Then, they’d have to pay any fees charged by the operator of the new marketplace. In Epic’s case, that’s 12 percent — a significant discount on its own, but a major addition once you factor in Apple’s costs.

Checking Apple's fee calculator, apps that publish exclusively on third party stores don't have to pay Apple any commission, just the core technology fee. That makes it a bit less crazy, and I don't think article mentions it. Epic could save itself a lot of money by just not using the App Store but complaining is much more fun for Tim Sweeney.