The article is about text messaging, not forum posting. The difference between the two is in the level of formality. When you're messaging your friend, you recognise that it's not a formal situation, so you're free to type casually. Conversely, when you're posting in the forums (especially Discussion (but not so much with clan boards)), you're engaging in a much less casual and much more formal context.

Obviously, this is a video-game forum with a lot of kids so there are people who type and spell like shit, even in this formal context. That doesn't disprove this theory though.

IRC links well with this topic. Nobody in IRC uses proper grammar. If you took people that type correctly on the forums and looked at how they typed in IRC, you'd see a big difference. People in IRC are closer and so they act less formally. The reason that we might perceive people who type properly in IRC and in texts as insincere or less friendly is because of that disconnect between the level of closeness and the level of formality. You're expected to not act so 'robotic' in such a casual environment. Makes sense to me.