Unit tests testing the API - if there is no API there then the level of unit it probably too low. The purposes of all tests is to say "No matter what this will never change" - while never is a bit to strong as you are allowed to make changes any API that your unit tests covers will be painful to change, both because the tests will also have to change and so you have nothing to guide you, and also because odds are you don't have good coverage from other tests (integration tests would catch issues, but you rarely have all cases covered).
Or to put it a different way, your unit tests should cover units that have a good boundary to the rest of the system. This should sound like a module, but there is reason to have module as a larger thing than your unit (most of the time there shouldn't be, but once in a while this is useful), and so while there is overlap it is often useful to consider them different.
Integration tests cover the API, but they do not test the API (well they often use some API as well, but the won't cover all your internal APIs.)
Or to put it a different way, your unit tests should cover units that have a good boundary to the rest of the system. This should sound like a module, but there is reason to have module as a larger thing than your unit (most of the time there shouldn't be, but once in a while this is useful), and so while there is overlap it is often useful to consider them different.
Integration tests cover the API, but they do not test the API (well they often use some API as well, but the won't cover all your internal APIs.)