The point more accurately is that coupling to the interface used to communicate with your collaborator I.e. If you were to refactor the way that the test object

The point more accurately is that coupling to the interface used to communicate with your collaborator I.e. If you were to refactor the way that the test object communicates with it's collaborator (without changing it's external behaviour) you are now forced to update the mocks. If the collaborator is only used within your test object and nowhere else in your system the communication between them doesn't need to be pinned down, the test should just test both of them at the same time. This is the difference between a white box test (where your mocking has to expose implementation details) and blackbox testing (where you can refactor without breaking tests).