Those aren't a list of symptoms, but instead an outcome what happens given typical depression symptoms coupled with teen behavior (and not knowing how to handle the feeling). Actually, social isolation is a fairly typical symptom of depression. Substance abuse and so on are often correlated as well.

Looking for the study itself (which I can't seem to find), but the article says something along the lines of "showing symptoms of depression". Symptoms of depression are often caused by depression.

No, it's not an insult at all. The biggest insult to depression sufferers is not recognizing it as a condition or implying that they are something other than depressed.

Edit: I'm sorry; perhaps you are simply not understanding the article. Is English your primary language?