whose rallying cry was "Rationality! Reason! Objective reality!"

Not to disagree with your main point (I've seen cultishness even in mathematics, where we really do have objective reality), but aren't those cults whose banner is Rationality in a better position than those who aren't? They may be just as cultish on the inside, but they have publicly accepted a standard that makes then vulnerable to criticism they cannot just dismiss. Wouldn't that make them a bit more honest?

Same point: are we more honest at overcoming bias, because we have a type of discourse that leaves us vulnerable to arguments of bias in ways we can't ignore - or do we just become more skilled at rationalising?