It’s not a pejorative word for any Stalinist. It’s a pejorative word for a specific type of Stalinist, ie, one that will back any oppressive regime that espouses itself to be Stalinist.

Via Wikipedia:

Tankie is a pejorative label for communists, particularly Stalinists, who support the authoritarian tendencies of Marxism–Leninism or, more generally, authoritarian states associated with Marxism–Leninism in history. The term was originally used by dissident Marxist–Leninists to describe members of the Communist Party of Great Britain who followed the party line of the Communist Party of the Soviet Union. Specifically, it was used to distinguish party members who spoke out in defense of the Soviet use of tanks to crush the Hungarian Revolution of 1956 and the 1968 Prague Spring uprising, or who more broadly adhered to pro-Soviet positions. The term is also used to describe people who endorse, defend, or deny the crimes committed by communist leaders such as Vladimir Lenin, Joseph Stalin, Mao Zedong, Pol Pot, and Kim il-Sung.