Thanks for the post. I do not work on that team so I can’t speak by their policies. However their policies are very well documented at https://developer.apple.com/app-store/review/guidelines/
I would recommend to go over them as they relying exclusively on those so a web-loaded content to provide the only exit from a full-screen view is a well-known anti-pattern in mobile development, and your concerns about resilience and App Store compliance are highly justified at your end.
While there isn't a single guideline that explicitly says "you must use a native close button on web views," this design pattern may triggers the policy under Guideline 4.0 (Design) and Guideline 4.2 (Minimum Functionality) then again, I think you should read it and to make sure your app does not go against those policies or any other in place.
The main reason to answer your post is about designing an app that follows a user interface that becomes a compromise as UX teams usually request views like this because they want an immersive, uninterrupted visual experience without the clutter of a standard iOS Navigation Bar? You can achieve their visual goal while maintaining native resilience, in my modest opinion.
I would suggest UI designers to jump into this thread and provide opinions on your requirements, as a developer I would suggest to keep the web view completely full-screen edge-to-edge, but overlay a native, floating UIButton (like a semi-transparent 'X' or a standard Symbol) in the top right or left corner.
If we rely solely on the webpage to provide the close button, any network blip or web error will permanently trap the user, forcing them to kill the app I think.
Albert
Worldwide Developer Relations.
