Again. There is an existence proof. I worked for a company in the healthcare space that was strictly business to business it was used for secure HIPAA compliant messages between doctors in a hospital network.
You couldn’t use the app unless you were a doctor who belonged to a clients network.
As far as not understanding what app to pick. There are plenty of companies including FB that have apps for the general public and apps for a subset of users.
That's also a thing with app store : the fact that someone else's app works one way doesn't mean a lot. Your app could get accepted for months and then all of the sudden be rejected after an update makes it go through validation again.
But that was the App store's rules, and people more or less learned to go with it. What worries me a lot with the recent news is that the lottery could now affect enterprise certificates as well.
Also, asking for a login at startup without providing a way to register via the app was against the store tos, iirc.