But that assumes that Apple is the marketplace in the first place.
The best way to think about this is in terms of accesories for physical products.
Unofficial aftermarket accesories are considered completely legal to make and sell, without paying a dime to the base product's manufacturer. Such should be the case with iOS. Apps are essentially "digital accesories".
This feels a lot like a case where the EU may have wanted Apple to lose all their controls but they didn’t put it in the law.
Apple can still charge 30% (perhaps minus CC fees). They can still force you to be a developer in good standing. They can still limit what apps can do via their APIs. Heck they might be able to give App Store apps extra privileges because the developer choose the App Store.
If they wanted iOS to look as open as Windows or MacOS they needed to specify that.
The problem is that, at least in Europe, the intent of the law is evaluated just as strongly as the letter of the law.
So, if we got this law that very clearly says it shall be easy to technically and effectively install third-party apps as a user, and also contains provisions that say third-party apps must get the same API access first-party ones get, it would be ridiculous for a court to reach the conclusion that it's acceptable for a developer to still be subjected to Apple's whims, except now they're being screwed inside their own infra!(tm)
The best way to think about this is in terms of accesories for physical products.
Unofficial aftermarket accesories are considered completely legal to make and sell, without paying a dime to the base product's manufacturer. Such should be the case with iOS. Apps are essentially "digital accesories".