So was their analysis of depressed people purely based on post/comments on mental health forums? To me, it makes sense that if you were going to one of these forums for the express purpose of getting help, of course you'd be using first person pronouns and negative-words, I imagine a lot of the posts would have something to the tune of "I've been feeling sad lately". Likewise, they compare this to formerly-depressed people on a forum explicitly for positive encouragement and found they used more positive-sounding words. The study seems to me to be analyzing whether people stay on topic in these forums more than saying anything about depressed people in general. While this would be much more difficult to accomplish and probably isn't realistic, I think it would be way more helpful to analyze how depressed people post on social media or via text or other "normal" conversation rather than how they interact when they go somewhere to talk about depression/being depressed.