It's also a relatively apt description of a small subset of left-wing groups. My assumption is that the parent comment is explicitly referring to this small subset.

Similarly "white supremacist" is often used as an insult on the inverse side, even for groups that don't believe in or desire white dominance. If someone actually does believe in those things, though, it's still a totally fine term to describe them.

Not really, you see the same term used in left wing circles to refer to liberals/synthetic left types.