>How are they even going to attempt to enforce compliance.
Per the judge's own words, Apple is allowed access to the company's financials and other information that would allow them to audit developers' use of third-party payment systems.
The relevant quote is below. She's not explicitly granting them that permission, she's saying that it would already be within their rights to make that the rule for participating in the App Store:
> As discussed in the findings of facts, IAP is the method by which Apple collects its licensing fee from developers for the use of Apple’s intellectual property. 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.
> In such a hypothetical world, developers could potentially avoid the commission while benefitting from Apple's innovation and intellectual property free of charge. The Court presumes that in such circumstances that Apple may rely on imposing and utilizing a contractual right to audit developers annual accounting to ensure compliance with its commissions, among other methods. Of course, any alternatives to IAP (including the foregoing) would seemingly impose both increased monetary and time costs to both Apple and the developers.
Per the judge's own words, Apple is allowed access to the company's financials and other information that would allow them to audit developers' use of third-party payment systems.