"Apple must allow the use of third party payment processors" does not mean "Apple cannot enforce any other rules against apps using third party payment processors". If they decide to go on a harassment campaign of petty violations against third party payment using apps or put undue requirements on them, those parties can complain to the courts and likely succeed. If the third party payment provider is stealing credit cards or obviously malicious behavior in some other area, Apple would likely prevail when pointing to that as the reason they banned them.
"Apple must allow the use of third party payment processors" does not mean "Apple cannot enforce any other rules against apps using third party payment processors". If they decide to go on a harassment campaign of petty violations against third party payment using apps or put undue requirements on them, those parties can complain to the courts and likely succeed. If the third party payment provider is stealing credit cards or obviously malicious behavior in some other area, Apple would likely prevail when pointing to that as the reason they banned them.