First, Epic is hardly a no name developer. But secondly a great deal of apps work like this both big and small. The limitation is you can’t make purchases though an app by say asking for credit card numbers or put purchase links to your website from the app.

There are well known and detailed App Store rules about this stuff, if you want details just look it up. For example all those 2 factor authentication app fits this model where it’s meaningless to download the app without a subscription.