Stalinist regime did plenty of outright genocides even by the most rigid definition of the word, even if you insist that it's not any mass murder, but one carried out on the basis of ethnicity. Technically, most of those were designates as "population transfer" (i.e. deportations) - but they were often carried out in a way that either deliberately caused casualties, or knowingly did nothing about the ones that were the inevitable byproduct of the implementation. For example, the mortality rate for Chechen and Ingush deportees was around 25% - that's literally hundreds of thousands of people; and in some cases, entire villages were massacred outright because deporting them was not deemed logistically viable. All those people suffered for their ethnicity and no other reason. If that's not a genocide, what is?

But anyway, originally we were talking about "the worst atrocities in the history of the world", not genocide specifically. So it's reasonable to assume that genocide in this context is used in a colloquial meaning - i.e. any kind of intentional mass murder by some group metric, not necessarily ethnicity.