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.
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.