The distinction is that you don't first declare a few all-encompassing interfaces and then their concrete implementations, but piecemeal interfaces for each API

The distinction is that you don't first declare a few all-encompassing interfaces and then their concrete implementations, but piecemeal interfaces for each API that takes those implementations - only the bits of the interface that the API in question needs. That makes it actually even easier to do unit testing, because for a narrow test you don't need to implement irrelevant methods.