It's noteworthy that these groups were mostly active in the 20th century as opposition to governments while right-wing extremists have been the government of many countries, including the US until recently.
Yes, I agree that it's very noteworthy. These groups generally appear in response to right governments. Which means I suspect we'll see more of them in coming years.
I am all for fighting far-right extremists. I am very against innocent civilians being killed, oppressed, or censored in the process.
If you're skeptical that leftism can cause violence, the wikipedia list of recent far-left terrorism groups may convince you otherwise: https://en.m.wikipedia.org/wiki/Left-wing_terrorism
And of course, there's the more classical examples: Maoist struggle sessions, the Red Terror, the Cambodian Killing Fields.
Extremism is dangerous. And it's concerning to me that this has become a debatable point.