Although the book revolves around Yali’s question, we do not really know much about him. This means that it is hard to know what he would be likely or unlikely to believe. We are told (p. 13) that he “radiated charisma and energy” and that (p. 14) “his curiosity was insatiable.” But we do not know much more about him. Therefore, we can only really guess as to whether he would believe Diamond’s explanation.
I would argue that Yali would believe Diamond’s answer. The main reason for this is that Diamond’s answer does not denigrate Yali and his people. Many answers to Yali’s question center on the idea that people like Yali are racially inferior or that their culture is inferior to that of white people. This would surely be insulting to an intelligent man who cared about his people and their culture.
I believe Diamond for much the same reason. Half of my heritage is not white and I certainly would have a hard time believing the idea that some of my ancestors were racially inferior to others. I also tend to doubt that some cultures are truly “better” than others.
Thus, I believe Diamond and I think that Yali would as well.