The answer to this can be found in Chapter 17, beginning on p. 350 in the paperback edition of the book. The different outcomes of this expansion show us how important farming is to the fate of societies.
Diamond says that the Austronesian expansion essentially conquered all of the Philippines and Indonesia but was unable to dominate New Guinea. The major reason for this was that people in New Guinea were farmers while those in Indonesia were still hunter-gatherers. The Austronesians were farmers. When they expanded, their more advanced civilization allowed them to defeat the hunter-gatherers, but they could not defeat the equally advanced New Guineans.
This is important to Diamond because he argues that farming makes societies strong and this can be seen as evidence for his thesis.