Arguably, it’s the definition of genocide at play here.
To qualify what I am going to say, I have a minor in History, with a particular focus on the 20th century.
There are moments in both the Soviet Union’ and China’s histories that are genocidal, or aren’t always considered genocide but probably should be considered it. Things such as the Holodomor, etc (I’m not going to argue if this is a genocide or not). As you mention, both nations likely killed more people than the Nazis did (although things such as the black book of communism should not be considered a credible source).
The difference is, neither the Soviet Union nor Communist China were founded on a platform of genocide. The Nazis were. The majority of people killed in the Soviet Union and China were not killed through genocide, they were mostly killed through political violence and state mishandling of resources. The intentions were just different; the Nazis cannot be understood without their desire for genocide, the Soviets can.
I agree, I’m just not sure what this is responding to. I didn’t intend, and I hope didn’t imply, that this was at play here. The Holodomor isn’t better than the Holocaust simply because one ended with fewer deaths, and I agree with what I think is undergirding your comment, that there’s no real way to “rank” genocides, they’re all bad.