Calling a person a protester is not a slander. “Insurrectionist” is in no way analogous to “idiot”. One is an allegation of criminal activity, which is slander if untrue. The other is an opinion on the quality of a person’s mind.
“The act or an instance of open revolt against civil authority or a constituted government.”
https://www.thefreedictionary.com/insurrectionist
Revolt:
“to experience disgust or shock”
https://www.thefreedictionary.com/insurrectionist
Those do not strike me as “wrong” if one is at a meeting that describes the bad government has done and the fact that I'm sick of it, too.
In other words, being called a murderer is clearly wrong, but a harmless occasion could have all of us called “Insurrectionists,” similar to being called “Christians,” in offense. We just aren’t used to throwing such a word around.