What is the significance of the differing outcomes of Austronesian expansion in Indonesia and New Guinea?
According to Diamond, the major significance of the different outcomes is that they show the importance of food production. One of Diamond's major themes in the whole book is the idea that cultures that develop agriculture early have a huge advantage over those that do not. Diamond argues that the experiences of the Austronesians in Indonesia and New Guinea show this.
In Indonesia, the natives never developed agriculture and were still hunter-gatherers when the Austronesians came. Natives of New Guinea, by contrast, had agriculture and settled together in densely populated areas. Because of this, the New Guineans had technology that was as good as that of the Austronesians. They were resistant to epidemic diseases. These things helped them to resist the Austronesian incursions.
So, the different outcomes were significant because they show how important it is to develop agriculture sooner rather than later.
The answer has to do with superior agricultural techniques. Diamond identifies a few key factors that determine the success of the Austronesians: these include superior tools and weapons, better boats and maritime capabilities, and higher population densities, made possible by improved food production. In the case of western Indonesia and the Philippines, the Austronesians replaced a hunter-gathering population because of their skill at producing food. In other areas, notably the southeast Asian mainland, they did not spread, because they possessed no technological advantage over the peoples already living there.
In the case of New Guinea, the native population had already developed agriculture, and native population densities were far greater. The Austronesians could not displace the native people, and were relegated to settling on coastal islands instead.