I think it's likely they are two different types of humility in Feynman's case, whether it's external vs internal humility or problem solving vs social humility or what have you. It does seem to me that Feynman had a humility in how he approached problems that was bolstered by confidence, or perhaps lack of social humility.

The advice he gives in the letter is to be humble in how one approaches which problems to solve. I think it's good advice.