Maybe Rand is referering to a specific situation where she knows Branden's thought processes and her statements are correct.

It was about arguing with collectivists (AKA people who were sympathetic to the USSR). Whether she was correct about communism being inferior to capitalism isn't easy to analyze objectively but in a sense history has validated her.

In that case, I wouldn't know. But if it's meant generally enough to be a rationality quote - if it's meant to explain why we get angry at dishonest people - then it's just an unsupported claim

It's supported by her personal experience. It is also largely supported by my own personal experience.

And equating dishonesty with both evil AND irrationality rubs me wrong. Rand believed that she's basically solved morality, and rationality only allowed one kind of morality, namely hers

Only partly true. Her morality acknowledges that man has the free will to think, but assumes that if he thinks honestly he'll come to many of the same conclusions that she does. The only real constraint in Objectivist morality is on the initiation of force.

I believe this is part of what locked her into an inescapable worldview, beyond correction and updating

This is an exaggeration.

(like what Branden wrote about how, once she decided that Reason's verdict on hypnosis was that it was bunk and had no foundation in reality, nothing could reach her on the subject)

This puts Rand within the general consensus of American psychologists. Branden also said that Rand updated on the effects of smoking marijuana.

I think it more useful to consider rationality (correct reading of reality and decision making) separately from values held.

Why? What if you notice patterns in values held and rationality? Should you ignore them?