“Why are you so convinced that there is Loshon Horo when talking about an anonymous online persona?”
1. If you are correct, then the person themselves couldn’t have been speaking l”h in the first place (since it was also about an anonymous online persona), so in that case, it would have been onaas devarim.
2. As you pointed out, some people do know who people are in real life.
3. I don’t think that you are right. In the coffee room, everyone has an identity and a name and is interacting with others. The fact that they use a different name here than they do elsewhere is irrelevant. By speaking l”h about someone, you are causing others to view them differently and to treat them differently at least in the Coffee Room.
And I think that I have actually seen this happen to some extent.