I grew up in Japan as well as in the U.S., and I'm inclined to agree with your take. This brilliant letter is the result of a cross-cultural exchange of ideas, philosophy, ways of life.
There's a strong Buddhist influence in Japanese culture, and in that value system, it's emphasized that selflessness is a virtue. "The nameless" is one of the phrases in this tradition, for pointing at the "ineffable mystery" - similar to how other religions like Christianity have words to talk about God indirectly, in deference, as "the Lord" or "the Highest".
In American culture, there's a strong tradition of individualism, self-reliance, and self-respect. Also, generally speaking, there's a sense of society as a competition.
On both sides, there are unhealthy extremes. The self-effacing approach can lead to loss of confidence and purpose, and to suppress the fullest expression of one's unique nature. And being "full of oneself" can lead to arrogance, blindness to one's own limitations, lack of empathy, and so on.
Personally, I read Feynman's letter and feel his humanity, the care and consideration for fellow thinkers, to encourage and support each person to value who they are.
Maybe the Japanese colleague did not need that advice, with a whole another understanding of the value and pride of being nameless - but, I tend to believe that Feynman sensed something, and had empathy to feel that this person needed to hear positive words to be confident in who they are and what they do.