Comparing the left to ISIS seems pretty obnoxiously rude to me.
I think they were comparing the fact that both ISIS and the people in this article (leftists) were bent on destroying art that they didn’t like. And you were directly rude to that person.
Plenty of people on the left compare the right to Nazism, which is equally obnoxious and rude. Not saying you did, but those types of generalizations aren't really conducive to proper discussion.