What I meant by "not possible" is writing both sides of the API at the same time. For example, you write a library for overlaying maps on video feeds, it is good if you are also writing the application that uses it. For example a drone controller. So in the early phase, you write the library specifically for your drone controller, changing the API as needed.
But sometimes, the drone controller will be made by another company, or it may be a project too big not to split up, that's the "not possible" part. And without a clear, in control use case, you have to make guesses, and writing tests can help make good guesses.
