Parenti's "non-falsifiable orthodoxy" statement explains it. It really is just reflexive, deeply ingrained, and completely unexamined anti-communism. It's not a real event to most of them, it's an article of religious faith that proves the righteousness of their anti-communist beliefs. Trying to tell them "no one died in the square, about 300 people including unarmed pla soldiers died in fighting between the pla and insurgents several blocks away" is like telling someone that the tears leaking from the virgin mary are a rusted out sewer pipe. You're not revealing the truth, or educating them. You're attacking their religious convictions that form part of the foundation of how their world works, and they react accordingly by shutting down and denying.