Try putting yourself in the other person’s shoes. Would you want another poster to say this about you? Even if you wouldn’t mind, think about it from the other person’s perspective. The other person might have friends who know who he is who will read the post, the other person might be more sensitive than you, etc.
The Rambam wrote (Hilchos De’os Perek 7): “Lashon Ho’rah is something which, if known to others, causes the person damage to his person or finances, or causes him pain or frightens him.” I guess the question here is whether an insult of a screen-name is considered “known to others”. I would say it is for the same reason that YW Moderator-42 wrote that some people might actually know who the person is. Even if nobody actually knows them personally, people do know them by their screen-name and insulting them can therefore still cause “damage” or “pain”. People may no longer respect that particular poster’s opinion anymore, etc.
This might be a bit of a stretch but, let’s make a comparison to insulting “The Rav of Shul Ploni” among a group of friends. Let’s say that nobody in the group knows the Rav personally or will ever actually meet him. They know him by a title. Yet, it can still cause him “damage” or “pain” if he ever finds out what was said even though he doesn’t know anybody in the group personally. If this particular Rav now issues a psak, the people in the group might treat it differently then they would have beforehand because of what was said. So too here, if you insult a particular member, it could be that nobody here knows who he is but if this member now posts an opinion, this opinion will be treated differently because of what was said previously and it could cause pain to that poster.