this is conflating a lot of concepts. there's nothing wrong with being rational with regard to the development of your own beliefs, and there's nothing wrong with using rationality to determine the best way to achieve your goals. when considering whether to be blunt with someone, you should consider your goals. if telling them the bare truth is going to offend them, and that's not in your interest, it's not rational to do so. being rational doesn't mean acting like a truth-telling robot.
All fair -- except the context is there's sort of a self-proclaimed "rationalist" community which is known for eschewing social norms to the point of seemingly deliberately over-considering controversial ideas as a reaction (e.g. certain racial topics, pandemic, etc). I think those people are the type we're talking about.
A truly rational person would potentially never call themselves "rational" because they would know most people find it an alienating term and kinda an unfair self-assessment.
