Applying the (current for any given time) criteria for depression requires a certain amount of training and skill, and the point form list of symptoms that you see as the first section in each diagnosis in the DSM-5 is not standalone. To mention one huge but often-overlooked example, something isn't a mental illness unless it significantly interferes with your life and functioning.

Not to say that people don't incorrectly make diagnoses all the time.

Many of these studies also rely on rating scales (typically HAMD or even PHQ9 for depression) that are a measure of depressive symptoms but don't in themselves have the power to make a diagnosis.