This is directly part of the underlying court decisions.
While the courts haven’t explicitly stated a percentage, and the initial judge questioned the percentage, it was made clear by both the district court as well as the appeals court that Apple can still charge a commission even when payment takes place outside of Apple’s IAP system.
The courts consider it payment for Apple’s IP that’s directly tied to the sales, not IAP:
“In essence, Apple uses the DPLA to license its IP to developers in exchange for a $99 fee and an ongoing 30% commission on developers' iOS revenue.”
The district court even went as far as to outright state Apple’s entitlement to a commission, despite hemming and hawing about the exact rate (and ultimately not making a decision on it):
“Even in the absence of IAP, Apple could still charge a commission on developers. It would simply be more difficult for Apple to collect that commission”
“Indeed, while the Court finds no basis for the specific rate chosen by Apple (i.e., the 30% rate) based on the record, the Court still concludes that Apple is entitled to some compensation for use of its intellectual property.”
“Apple is entitled to license its intellectual property for a fee, and to further guard against the uncompensated use of its intellectual property. The requirement of usage of IAP accomplishes this goal in the easiest and most direct manner, whereas Epic Games' only proposed alternative would severely undermine it. Indeed, to the extent Epic Games suggests that Apple receive nothing from in-app purchases made on its platform, such a remedy is inconsistent with prevailing intellectual property law.”
“Suffice it to say, IAP is not merely a payment processing system, as Epic Games suggests, but a comprehensive system to collect commission and manage in-app payments.”
The appellate court echoed these sentiments, if not outright making stronger statements about this, while at the same time complaining between the lines that the district court wanted its cake and eat it too by insisting that the anti-steering provisions are not kosher while simultaneously stating that it would be too cumbersome for Apple to retroactively audit sales to collect their commission.
Either way, the long and short of it is that Apple collecting a commission from developers using third party payment processors has the blessing of the courts.
Even when the district court in particular isn’t entirely happy with the rate of the commission while simultaneously not willing to make an official determination on the rate because Epic never fought the rate, rather the existence of the commission itself.
Now that SCOTUS has declined to look at it, this situation, including the blessing to collect a commission even when using third party payment processors, is the law of the land.