I'm not sure that's true. Stipulating that it was "the greatest problem", how could it be the defining characteristic considering all the other historical instances of mass murder?
Violence and extreme social distress, public humiliation sessions, public beatings were meted out rather liberally. A specifically pernicious aspect of Mao's strategam was turning generation against generation. These are the defining characteristics of Chinese Communist Party's Cultural Revolution.
The public humiliation sessions are what I think most people that didn't study it think of, combined with the violence. They're what I vaguely think of; they're what you mention, and I'm confident they're what most people think of. The concept of such sessions is something that found its way into Western fiction (e.g. 80s SF) clearly inspired by the historical period. I don't recall a specific example, but this is something I always assumed was part of popular culture at some level.
While we're on that topic (popular impressions of Chinese history), a lot of people died during the "Great Leap Forward" too, but again, I don't think the average uninformed opinion of what defines it is "mass murder". When I think of that, I think of peasants trying to make steel and other social disruption.
The Cultural Revolution is a poor analog in other ways as well (I mean, upon examination the comparison to our current moment doesn't hold up at all and it's boring to discuss) but the large number of dead people seems like an especially important indicator that perhaps what's happening today is not as serious as all that, and that the author is engaging in some pretty extreme hyperbole. (which is their right of course, blah blah)
Sure, but the notions of "re-education", "self-criticism" and the struggle-session as a form of public humiliation where someone is forced to admit their "crimes" before the "people" are a distinctive part of leftist social strife and oppression. And it seems quite relevant to point out that these social practices have led to mass murder in at least one instance that we know about, where they were promoted in an extensive "grassroots" campaign and thereby became widespread. We're not talking about willful and intentional physical purges of intellectuals ala Pol Pot's Khmer Rouge, where your contention that mass murder is the defining element of it all would be far more on point.