The problem is many people cannot see that Apple does use anticompetitive behaviour with it's app store. If an app developer designed an app and made it free for all, Apple would not have a problem with that. The app stays on the store for many years, the app gets 1000's of downloads, still no problem. The app developer decides to make an update that allows players to purchase in game items, a feature to help keep the interest in the game alive. The app developer see's there are numerous pay systems available as well as Apples pay system and thus approaches one of the companies requesting the use of their pay system, because the app developer feels that company offers them the best option. The app developer codes the app accordinging and Apple immediatly bans the app for rule breaches because they did not use Apple's pay system but somebody elses and tells the app developer that they must use Apple's pay system if they want to be unbanned.
This people is anti competitive behaviour and Apple will get into trouble for it, regardless of how Apple writes it's rules.
But thats NOT what happened with Epic.
In your scenario however:
The developer should NOT have approached any other payment company if they want the items to be available to purchase from WITHIN the app.
The developer COULD charge for those items OUTSIDE the App Store. They cannot however show how to obtain said items within the app or link to alternative payment methods - its on their own head how they market those paid add-ons to their customers in order to get enough people willing to pay the items
externally WITHOUT violating the terms of their developer agreement.
Email marketing, telesales, viral marketing, billboards etc are all avenues they could invest in to keep 100% of the profit .. if they are lucky they might just get a tiny percentage of sales conversions vs IAP , but you might easily spend just as much as the 30% and with a much bigger headache, if you have a captive app, with a captive audience willing to jump though a few hoops you might have a chance at success.
Advertising and Marketing costs money - ALOT of money, Apple decided 30% was their commision in order to convert your free customer to a paid customer via simple one-click IAP's or in the case of subscriptions 15% after a year.
I'd warrant that for every additional step you have to click though to purchase via an external source you are almost 100% closer to losing that customer, at least in the micro transaction market.
IMHO There is HUGE value to the developers in the way Apple's IAP's currently operate, especially when it comes to micro transactions and subscriptions which are often made without too much consideration (unlike buying a complete app or game).
So pray tell, why should Apple, or any other Appstore, be willing to let anyone advertise those add-ons from within an app without making a dime ?