Third party payments have always been possible to implement in iOS apps; I've done it myself. What happens, however, is Apple rejects your app during the review process if the payment is for something they deem should be an in-app purchase or subscription (physical purchases are fine). Without that review process, you don't have to use Apple's payment process and can use Stripe or Braintree or whatever you want, and bypass Apple's 15-30% commission.

> The regulation doesn’t allow you to mmap w^x willy-nilly

Yesn’t; I don’t think the regulators would look kindly upon Apple restricting third-party JITs, since it’d give Safari/WebKit an unfair competitive advantage over other browser engines on iOS (something I believe the DMA is also meant to open up.)