It was raised as a legitimate concern in an interview Theverge had with the Epic CEO, Tim Sweeney.This sounds like pure fantasy. Not a chance Apple gets a 30% cut of sideloaded apps. It will rightfully be 0%.

Epic Games CEO Tim Sweeney thinks “every politician should fear” Apple’s power
“I think it’s incredibly dangerous to allow the world’s most powerful corporation to decide who is allowed to say what.”

The judges were clear that even if the store was opened and alternative payments were allowed, Apple could still charge a fee. They’re doing this in South Korea, for example, where they’re still taking commission of 26 percent. And with Xcode, they could literally just charge you to run code, to run binary on their phone, no matter what. So I’m curious what happens if Apple is forced to open up distribution but Epic and other developers, they still have to pay the potentially double-digit percentage for in-app payments somehow. Is that enough for Epic?
In summary, he himself isn't entirely sure he will be able to get out of paying said fee, which would pretty much tank the viability of offering the epic game store as an alternative on iOS. He will certainly fight the case in court if it ever came to that, but the outcome is far from certain.