Some parts of Diamond's argument in the Epilogue are convincing, but other parts are less so.
The most convincing part is Diamond's argument as to why the effect of things like "great men" on history does not disprove his theory. Diamond argues that "great men" can have an impact on history in the short run (things like who won WWII) but that they cannot have an impact on the long run (things like why Europe came to dominate the world). This seems eminently logical because individual people can surely have only a very little impact on the course of tens of thousands of years of history.
The less convincing part is Diamond's explanation of why Europe overtook China for world dominance. Diamond explains this by pointing to China's isolation and unity. My issue with this is that Japan, too, was isolated and unified but reacted very differently than China did when confronted by the Western world. Japan is still economically much stronger (on a per capita basis) than China is. This seems to indicate that something (great men or culture perhaps) has had a major impact on the relative positions of China and Japan in the world over the past 150 years or so.