You are right that we cannot know and understand the life of every individual in a group, but we may observe typical or aggregate behavior, and we may seek reasonable inferences.
Tankies express a general pattern of behavior that is bad faith.
They quote passages instead of explaining from personal comprehension. They attack individuals against an opportunity to discuss ideas. They defer to doctrine instead of reasoning independently. They anchor to absurd lies about anarchists. They lie and deny instead of admitting to problems. They rely on disingenuous rhetoric such as the motte-and-bailey fallacy.
Such observations converge on a pattern of anchoring to convictions for reasons that are unrigorous, prejudiced, and generally misguided.