Whether cossacks, especially during the late Empire were an ethnic group seems to be quite blurred.

Doesn't stop anyone from putting it in the category of genocides perpetrated by the communists. Similar to Holodomor, where most of the people absolutely certain that it was a deliberate mass-killing of Ukrainians, are those that just want to up the number in their "Black Book of Communism".

Kulaks certainly weren't an ethnic group.

Genocide is quite a flexible term and is oftentimes extended to all mass-killings of groups of people sharing similar characteristics (i.e. used for example as a synonym for the shittiest term classicide).

Anyways, my point wasn't "genocide good and necessary actually", more like "not every authoritarian measure any ML state took was purely genocidal". That's caving to propaganda