Respectfully, you are wrong. The realities of human biology don't all fit into neat little boxes.
With no evidence presented you dismiss the entire body of scientific evidence which backs the rather basic idea that depression, like any other biological process can be caused by a wide variety of sources not just external factors.
Being very sad because of an internal reason and clinical depression are not the same thing though to an outsider they may appear the same. Much like being overweight because you eat too much and sit all day isn't the same as being overweight because your thyroid isn't functioning properly aren't the same thing. What you're doing here is the intellectual and moral equivalent of blaming people with an underactive thyroid for their weight gain because you can't tell the difference by looking at them.
You're right on one point depression is the symptom not the disease and there's huge room for misdiagnosis here. Likely our understanding will ultimately split clinical depression into many different diseases that cause it as we learn more but its simply foolish to dismiss the disease possibility out of hand.