I'm not educated, let alone a historian, but there do seem to be some parallels here and it seems like the most disparate factor would be the very specific oppression of Jewish people. But the Soviet mass murders involved the death of a huge number of 'undesirables'; most just happened not to be Jewish. They were thrown into unspeakable conditions of torture, murder, starvation, etc. so I can see why Snyder would see them as similar.