A lot of the arguments I've seen "tankies" make is that they're the ones upholding the true non-revivisionist ideals of Marxism-Leninism. What I'm about to say also isn't my own beliefs, but I've looked into what they have to say, not deeply, but often it comes down to pointing out western propaganda against communist countries. For the DPRK for example, they say the famous space pic was taken during a black out, the famine of the 90s was caused by the collapse of the Soviet Union, the detention camps aren't real, etc.

For the USSR, particularly Stalin, it often focuses on bashing Trotskyism as a conspiracy theory that undermined how a lot of the people view the country, especially the west, saying the purges were necessary to maintain the stability of the USSR so as to not have it fall into civil war. Then anything Khrushchev and beyond was the result of revisionism and literal US intervention.

Again, these aren't my beliefs this is just the jist of what I've gathered from trying to understand these people.