It’s a combination of honestly believing that Stalin and Mao lead genocides against their own people, and believing that people who defend China are shills for the Chinese government. Anti-Communism’s a powerful tool for making supposed Anti-Fascists side fully with out-in-the-open Fascists.

While Trump was president these people would go on and on about how ‘evil’ he was, then nod along in joy hearing him talk about how ‘evil’ North Korea and China are. Because, ultimately, these people are incapable of being critical of the sources for the things they believe in.