There is some truth to what this guy says, although he is mostly wrong.

Firstly, you are confusing feeling depressed with having clinical depression.

if you find your current situation in life shitty then it is perfectly reasonable to respond with depression

True, for "regular" depression. Not necessarily clinical depression. If you read Feeling Good by David D. Burns, you'll learn that the author has treated a lot of people with clinical depression, some of them in shitty situations, but also some very successful, very achieved people that had no apparent reason for feeling depression but were still clinically depressed.

"Regular" depression is not an illness. Clinical depression, however, is.

Diagnoses of mental illness are only the subjective moral judgments of psychiatrists that another person's alleged behaviors/feelings are wrong.

This much is true enough, and will probably be that way until doctors find a better way of diagnosing mental illnesses.

Unlike real diseases there is no underlying measurable damage or infection, just the assertion that thinking or acting one way is undesirable

Mostly true (for now), except the "real diseases" part. In the old days, mental diseases weren't considered "real" diseases. However what's considered to be a disease or an illness, and what's considered to be a healthy person, as defined by the health industry, has since changed. A mental illness is now considered very much a real illness, just like, I don't know, the flu or whatever. You can disagree if you want, but if so, know that your definition (the old, now considered antiquated definition, no less) isn't likely to catch on by posting it on Reddit ;).