Hi Marc,
That statement on page 24 of the PDF you shared above is incorrect. We apologize for the confusion.
App logins are only for accessing public resources. The app developer may want to expose public resources that use credits. App logins can be used by the developer against these public services if they wish to bear the cost of the service and not pass that on to their end user.
(See Implementing App Login | ArcGIS for Developers )
As for sharing access to private resources on ArcGIS Online to a relatively high number of users, there is some flexibility here.
(See License your app—ArcGIS Runtime SDK for .NET (WPF) | ArcGIS for Developers )
Basic License example:
Let's go with the number you gave, 300 users. Your Runtime mobile app is licensed at the Basic level. Some of your users may already have an ArcGIS Online account, as Level 2. They log in to the app using their Named User credentials, and off they go. For those that don't have an ArcGIS Online Named User account, your app has a Deployment License string baked into the app, unlocking it for them at Basic. The same app can work for each use case.
Lite License example:
Some of your users may already have an ArcGIS Online account, as Level 1 OR 2. They log in to the app using their Named User credentials, and off they go. For those that don't have an ArcGIS Online Named User account, your app has a Lite license string, obtained from your developer subscription, baked into the app.