[Haskell-cafe] Cabal confusion. Modules not being exposed still accessible for testing.

I use reactive-banana https://github.com/HeinrichApfelmus/reactive-banana/blob/master/reactive-ban... as an example. My question is how can Reactive.Banana.Internal.Combinators be testable from Reactive.Banana.Test.Plumbing? My misunderstanding is that only modules named in the exposed-modules stanza could be imported. Could someone clear up my misunderstanding?