I think “genocide” has the specific meaning that it’s an attempt to wipe out an entire people. It is mass murder, but with an identified demographic target. It’s not saying anything about being a lesser or greater offense, but merely describing the nature of the objectives.
Stalin let Hitler do his dirty work for him.
I believe that once Hitler invaded the Soviet Union, and Stalin decided which areas he would defend, he intentionally allowed Hitler to have the areas where the most Jews lived, knowing Hitler would take care of Stalin’s “Jewish Problem” for him.