Sometimes there are well-defined processes for performing a task, and that task is performed in only one place in the system. Therefore the details of the process can be kept class private inside of the only consumer. That doesn't mean that the processes should never be tested. If the task is cumbersome to set up or runs slowly then there is good reason to test the internal parts of the process which can be tested with dozens, hundreds or even thousands of permutations of input data cheaply and efficiently. Always relying on the large-scale tests to hit every combination of inputs for a well understood subroutine can be inefficient.
You could make the argument that this well-understood process could be broken out into its own class/package/module and tested with its own public interface, but if there really is only one consumer then that's kind of a strange trade-off to make in many cases.