The answer to this can be found in Chapter 16 of the book. There, Diamond discusses why people who spoke Sino-Tibetan languages came to diffuse themselves all across East Asia until essentially nothing was left of the original populations in places that originally had ethnically different peoples.
The reason for this total diffusion was environmental. Diamond argues that there were two important environmental factors. First, China has long east-west rivers that allow people to diffuse along them. Second, there is little in the way of geographical barriers to north-south diffusion. For these reasons, people were able to spread in all directions.
East Asia's relatively open environment with few barriers allowed the population of Sino-Tibetan speakers to diffuse easily across the region.