Okay, not many here know all the ins and outs of what is possible and what has happened, so a bit more info...
iOS was designed and built with security and sand-boxing in mind from the start.
Apps built with a regular developer account are signed with a certificate and profile. These apps can be installed locally to iOS devices on a Mac (for testing) and submitted to the app store for distribution. This type of account costs £99 a year and is the type of account Epic uses for Fortnite. Apple has not threatened to terminate this account, doing so would likely cancel all it’s certificates and disable all installations of Fortnite on iOS devices thereby rendering all in-app purchases extinct/gone (which may be a legal issue in and of itself and why Apple hasn’t done this - yet).
Apps built with an enterprise account are similarly signed with a certificate and profile. These apps can be installed locally to devices, can’t be submitted to the app store but can be distributed either via the web or an MDM solution within an organisation, not publicly. This type of account costs £250 a year. This is the type of account (to my understanding) that Epic International Group violated the terms of that resulted in Apple threatening to terminate. I suspect that whilst the enterprise account is meant for internal Epic use only, Epic has been using it like FaceBook did theirs, for devices outside of their own organisation. If so, Apple should have every right to terminate this account, it has no impact on Fortnite. This is where I think the court was very wrong.
It is actually quite a straight-forward matter to build for example a web-based app-store for native enterprise iOS apps as the certificates/profiles allow this and this is what MDM solutions provide. I worked at a company that did just this for their own internal enterprise apps (built their own internal web-based MDM solution).
So it would in theory be possible to build an app that was an app-store itself (containing links to enterprise apps hosted on the web) and submit it to the app-store. Of course Apple would never accept such an app into the app-store, as not only would the app-store app be breaking the ordinary developer agreement, the apps linked to and distributed via an enterprise certificate/profile would be violating the terms of their enterprise agreement(s).
If Epic had had the foresight they could have done just this and demonstrated that it was possible but they went for low-lying fruit instead (hiding features they could turn on/off or customise remotely). Or maybe they daren’t risk their enterprise Unreal account?
Now, Epic have expressed explicitly their desire to build their own app-store app, and have demonstrated their willingness to break agreements/app-store rules, so Apple should take note that Epic may submit a new app that has a hidden feature opening a new Epic app-store with links to enterprise signed apps. Epic could even write an enterprise epic app-store app and distribute it via the web from the Epic website, bypassing the app-store entirely, just like they did on Android but got no traction with. I can imagine them doing this just for fun, to bait Apple.
I believe this danger is real so Apple should terminate the ordinary Epic developer account immediately, even if this implements a killswitch on Fortnite for all installations. The court has not prevented Apple from terminating the ordinary developer account, the court has only temporarily prevented the enterprise account being terminated. Unfortunatley Epic could use this enterprise account to create and release an Epic app-store app via the web, this is a scenario which the court has enabled and which the court has potentially prevented Apple from stopping.
Apple could probably more simply just revoke specific certificates without terminating entire accounts, so that makes the waters even more muddy.
Hope that clarifies a little for some.