To play it solo in the manner intended, yes, you'd need the app. You can check it out here: Fable on your Table if you are curious as to what it entails.
But you could also use Fable on your Table as a miniatures skirmish system for a TTRPG experience, and make up your own scenarios. The app is there to enable solo play, but the system itself doesn't rely on the app to work, any more than any other TTRPG combat system does. For instance, you could write up your own adventures and share them with friends, and it would work perfectly fine.