I was under the impression that 'leftist' (like SJW) is a derogatory term used for people who would likely call themselves 'liberals' or 'progressives'. Just like I've never seen someone unironically call themselves a 'fundamentalist'.
The intent of the OP was derogatory. Doesn't matter what the recipient thinks.
I think leftist and SJW are not necessarily considered derogatory by those who identify as those things.
It doesn't detract from your point, but.. I've been around hundreds of people who use the term fundamentalist proudly as sort of an advertisement for their brand of christianity. They looked askance at other flavours of christianity, believing those to have drifted from the fundamentals of the religion.