No, it’s not an insult because it’s referring to a large group of people with a political agenda. It’s not different from insulting Republicans or anti-gun activists or … Those are all large groups of people with a strong stand on an issue, some of whom happen to be posters.
We don’t have rules against insulting such groups (or any outsiders, for that matter); our rules are against insulting other posters. Yes, I agree that there’s some slight awkwardness – insulting “all Democrats” is insulting some of our posters. But it’s usually a question of the thrust or implication. I see Left Hand’s comment as being addressed at a large group with a stance on an issue; I saw your comment as being addressed only at one poster.