I would argue that the premise of this question is mistaken. In the Epilogue, Diamond is not really trying to respond to “objections to his history.” When he talks about China or about the “Great Men” approach, he is not trying to rebut objections. Instead, he is trying to talk about where further research might go. Before discussing China, the “Great Men” theory, and other such things, Diamond says the following:
Naturally, a host of issues raised by Yali’s question remain unresolved. At present, we can put forward some partial answers plus a research agenda for the future, rather than a fully developed theory.
This shows that Diamond is not trying to rebut critics.
I do tend to agree with Diamond, though. I certainly believe his main argument about the Great Man theory. It is clear that individuals do not have the power to change the huge forces of history. No individual could have made the Native Americans strong enough to resist the European invasion. I find his theory about China convincing. It does make sense that China’s geography made it more unified and therefore less able to adapt. Economic theory tells us that a lack of competition leads to a lack of innovation. This makes me tend to believe Diamond’s idea that China fell behind due to a lack of rival countries.
Again, though, Diamond does not claim that he knows these things for sure. He is just suggesting directions for future research.