I've had a couple of apps in the iOS and macOS stores that couldn't be accessed unless you paid for a service outside the app.
In fact, when submitting an app for review you can provide a user and password so that Apple can log in and test the private app.
>3.1.3(a) “Reader” Apps: Apps may allow a user to access previously purchased content or content subscriptions (specifically: magazines, newspapers, books, audio, music, video, access to professional databases, VoIP, cloud storage, and approved services such as classroom management apps), provided that you agree not to directly or indirectly target iOS users to use a purchasing method other than in-app purchase, and your general communications about other purchasing methods are not designed to discourage use of in-app purchase.
I think this was allowed because as someone else mentioned there are some exceptions to this rule. This was an education app which gave access to some digital books.
It isn't. Go read the guidelines again.