Also, depends on what you define as “left extremism”.

Is political inclination a 1-dimensional axis? I'd argue no.

I think "left extremism" doesn't really work as a descriptor, either. I would say "authoritarian left" is probably more accurate, though that has its flaws too. Those who believe in the supremacy of the authority of the revolutionary party, perhaps?

In a word, "statists". Or "party-purists", maybe.