The problem that I see with self-backed apps that Onyx make is that they're not following standards (and whichever they choose works worse than the common way of doing things on android)
Speed Software FMs (explorer and root explorer) show how most apps should work. They use methods as showcased in android basic samples:
https://developer.android.com/traini...#StartActivity
Other software, like CC, build their own file picker, but work as intended as they're still relying on PackageManager and intent features.
What Onyx does could be whatever. They did broke most guidelines (physical back button instead of home, custom key mapping...) and I have a few reports they're not using intents/startActivity on a few things (namely to start dictionary app when you query a definition).
A good way to discover if intents are used at all is to install
Intent Intercept from Fdroid, enable it and see what happens when you long press a book on your library.