It looks like it is in the App Store, so I guess my question is already answered - but I wonder if they had trouble or concerns getting it accepted.
Section 2.5.2 in "App Review Guidelines" (https://developer.apple.com/app-store/review/guidelines/):
2.5.2 Apps should be self-contained in their bundles, and may not read or write data outside the designated container area, nor may they download, install, or execute code which introduces or changes features or functionality of the app, including other apps. Educational apps designed to teach, develop, or allow students to test executable code may, in limited circumstances, download code provided that such code is not used for other purposes. Such apps must make the source code provided by the app completely viewable and editable by the user.
Yeah there was definitely some back and forth about it before we were eventually approved.
In a sense, this isn't very different from what React Native does (run interpreted code that calls out into native code), just with Swift instead of JavaScript. There used to be JavaScript-specific requirements in the guidelines, but that has been loosened since Swift Playgrounds was released. Now there are Python IDEs, Jupyter Notebooks, and other apps running arbitrary code in the App Store.
The root of these restrictions is to try and prevent people from using dynamic code to bypass App Store reviews. If you're planning to change the functionality of your app then you must submit it for review.
yeah, this is the exception since it is a dev tool. Not sure Apple will approve a regular app doing it.