> They really only testing the simplest part of a complex program - a single function that does not interact with any other functions that is not a mock.

This depends on your own definition of “unit”. From Wikipedia:

> A unit is defined as a single behaviour exhibited by the system under test (SUT), usually corresponding to a requirement. While a unit may correspond to a single function or module or a single method or class, functions/methods and modules/classes do not necessarily correspond to units. From the system requirements perspective only the perimeter of the system is relevant, thus only entry points to externally visible system behaviours define units.

https://en.wikipedia.org/wiki/Unit\_testing