They are counting all Russsian casualties from WW2 as Stalin's fault in that count. It seems pretty absurd to blame Stalin for Hitler's genocidal war on the Soviet Union.

Yeah, there are a number of references to be found.

https://www.ibtimes.com/how-many-people-did-joseph-stalin-kill-1111789#%3A%7E%3Atext=Most+reputed%2C60+million.

The only two claims that are close are:

In his book, “Unnatural Deaths in the U.S.S.R.: 1928-1954,” I.G. Dyadkin estimated that the USSR suffered 56 to 62 million "unnatural deaths" during that period, with 34 to 49 million directly linked to Stalin.

and

In “Europe A History,” British historian Norman Davies counted 50 million killed between 1924-53, excluding wartime casualties.

Aleksander Solzhenitsyn also wrote on the matter. His estimate was 60 million in the gulags alone.

Solzhenitsyn collated what was effectively an oral history of the GULAG, not a serious academic study of the numbers, which were largely unavailable until the Soviet archives were opened after the Cold War.