You may disagree, but it is a pretty widely agreed upon distinction. It’s a symptom of the issue of a fractured left wing. The left leaning communists wanted to distinguish themselves from the left leaning capitalists, so they started calling themselves leftists and not liberals.
It’s just a bunch of different labels, it’s not really set in stone or definitive. I totally understand why you disagree.
My point really was just that a conservative calling someone a communist isn’t insulting to a lot of self described leftists because they are communist, it’s mostly just considered an insult to a conservative. Kinda like calling an alt right person a Nazi is an insult from a leftist, but plenty of alt righties wouldn’t be insulted because they are (jk but not really but jk)