I believe the term “tankie” comes from the idea that they believe the guys in the tanks at Tiananmen Square were in the right.

From the Wiki:

The term "tankie" was originally used by dissident Marxist–Leninists to describe members of the Communist Party of Great Britain (CPGB) who followed the party line of the Communist Party of the Soviet Union (CPSU). Specifically, it was used to distinguish party members who spoke out in defence of the Soviet use of tanks to suppress the Hungarian Revolution of 1956 and the 1968 Prague Spring

https://en.wikipedia.org/wiki/Tankie

The term "tankie" was originally used by dissident Marxist–Leninists to describe members of the Communist Party of Great Britain (CPGB) who followed the party line of the Communist Party of the Soviet Union (CPSU). Specifically, it was used to distinguish party members who spoke out in defence of the Soviet use of tanks to suppress the Hungarian Revolution of 1956 and the 1968 Prague Spring, or who more broadly adhered to pro-Soviet positions.[6][7] The term has extended to describe people who endorse, defend, or deny the actions of communist leaders such as Vladimir Lenin, Joseph Stalin, and Mao Zedong. In recent times, the term has been used across the political spectrum and in a geopolitical context to accuse individuals of having a bias in favour of anti-Western states, authoritarian states, or states with a socialist legacy; Belarus, Cuba, China,[8][9] Iran,[10] Nicaragua, Ba'athist Syria, North Korea, Russia, and Venezuela serve as prevalent examples.

The tanks they supported & were named after were not Tiananmen Square, but actually 30 years earlier when the USSR crushed the Hungarian Revolution of 1956.