Where do you find in that link the suggestion that rationalists should be less confident?

"Beware lest you become attached to beliefs you may not want."

"Surrender to the truth as quickly as you can."

One who sees that people generally overestimate themselves, and responds by downgrading their own self-confidence, imitates the outward form of the art without the substance.

Not necessarily. If there is no known way to correct for a bias, it makes sense to do the sort of gross correction I described. For example, if I know that I and my coworker underestimate how long my projects take but I'm not aware of any technique I can use to improve my estimates, I could start by asking my co-worker to do all the estimates and then multiply that estimate by two when telling my boss.

Is there are known way of correcting for human overconfidence? If not, I think the sort of gross correction I describe makes sense from an epistemically rational point of view.

Someone who "feels" it is their "duty" to do something is someone who already does not want to do it, so by definition the second has motivation and the first does not. But these are imaginary people and this is fictional evidence. A real answer to the rhetorical question might be found by surveying mathematics students, comparing those who stay the course and those who drop out. I do not know what such a survey would find.

Do you deny that believing you had the answer to a mathematical problem and only lacked a proof would be a powerful motivator to think about mathematics? I was once in this situation, and it certainly motivated me.

Someone who "feels" it is their "duty" to do something is someone who already does not want to do it, so by definition the second has motivation and the first does not.

The way I used "duty" has nothing to do with disliking a thing. To me, "duty" describes something that you feel you ought to do. It's just an inconvenient fact about human psychology that telling yourself that something's best makes it hard to do it. Being epistemically rational (figuring out what the best thing to do is, then compelling yourself to do it) often seems not to work for humans.