The idea wasn't to use third party processors as an argument to evade the fee, it was the other way round. Front and center Epic wanted to get rid of or strongly lower the fee, and as a consequence developers would probably have had to use other processors.
The judge shut this down by legitimizing Apple's 30%, which as a consequent allows Apple to collect that money however the dev processes their customers, third party PSP or not.
That only works if developers actually could save significant money by selling through alternate storefronts, which was never going to be possible because Apple was always going to pick something approximately market rate as their valuation for the payment processor portion of the fee.
[0] https://news.ycombinator.com/item?id=39014642
For example:
> It seems easy to kind of shrug at this, but this does seem quite significant because of many mobile apps are 'free to play.' Apple pocketing 30% of all of these transactions, and forcing users/devs to go through Apple, is a major part of its revenue. And all of those apps now have the option to direct users to alternative payment methods, where they can both charge users substantially less and make more profit doing so.
So charge developers for the service of using the App Store, perhaps with a per-download cost. To "give away" the service of the App Store only to recuperate your costs (and then some) via payment processing is just daylight robbery.
Charging customers by some other correlated proxy is not robbery; it’s just practical. Which is exactly why the court had no issues with the arrangement.
Given that this is the US legal/political system, that question is being fought out in various proxy fights, but by looking at the largely equivalent situation in the EU and the DMA, you can see what’s really at stake here.
That part of this outcome was inevitable, and I don't think any court will rule against it. It was even anticipated in the original ruling:
> First, and most significant, 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.