From the article it seems their key argument is that depressed people are identified by inconsistent heavy Internet usage, as opposed to just heavy Internet usage. If this were applied to heavy Internet users that are also parents, have a steady job, own a home, etc, they would likely show up as depressed. In reality they can only be "heavy" Internet users when free time exists.
Also, I wonder if the test is properly separating people who are depressed from those that are just introverts. Introversion != Depression. I would challenge that a lot of these people that are being categorized as depressed are actually just introverts trying to deal with the very extroverted college/university lifestyle. It takes a lot of energy for an introvert to deal with all those extroverted people, and a perfect way for them to unwind is to have some quiet time downloading, chatting, surfing, etc. Catching up on all those things that they feel they missed because they were feeling they must go out and be social. Once they've recharged they go back out and be social for a while. That could explain a lot.