In my eyes an ideal library should not contain state, internally allocate (unless very obviously), or manage processes. The application should do that, or provide primitives for doing it which the library can make use of. That makes applications and libraries very very different in my mind.

Yes true, the one difference might be that you don't expect other consumers with a different approach to IO to use your internal libraries, although it does help you if you want to change that in the future and the testability is still useful