Another thing that helped was their habit of hunting sea mammals. The shell mounds show abundant evidence of this type of hunting including sea mammal bones. They used harpoon heads including some probably of the toggling type, which have survived until modern times in the Philippines and New Zealand. A toggling harpoon has a detachable head attached to a line or cord.
The people also supplemented their diet by hunting and by raising domestic animals. They had chickens, pigs and dogs.
Many of them practiced horticulture -- evidence of which goes back to at least 15,000 BC in this region. And there is also evidence of sugarcane and rice agriculture.
The dates on the start of rice agriculture are rather controversial. Oppenheimer has a good discussion on this in Eden in the East. The earliest dates go back to 12,000 years ago at Spirit Cave and 9260 years ago at Sakai Cave on the Malay Peninsula. It is difficult though to tell wild rice from domestic rice just by looking at it.
The domestication argument is strengthened by the fact that other plants found at Spirit and Sakai caves were among those later domesticated in Southeast Asia.
Whatever the earliest dates for rice, the Nusantao that had reached South China definitely were planting this crop.
These shell mound people used ground-edge tools of both shell and stone. And a new discovery at Balobok Cave in the southern Philippines dated to 5340 BC suggests they also used fully-polished neolithic tools.
One thing we should remember in studying Southeast Asia is that a Neolithic or Metal Age "revolution" does not mean the same thing here as in other places. There are cases of "Stone Age" people surviving in this region to the present-day. The controversial Tasaday are one well-known example, but there are many other less controversial ones. "Mesolithic" Hoabinhian sites have been discovered surviving in regions that appear to had already moved into the Metal Age. Keep this fact in mind.
Here's a good summary of the Nusantao:
During the third and last rapid rise flood a Hoabinhian-like people that built shell mounds began migrating southward into insular Southeast Asia. These people certainly practiced horticulture and possibly agriculture. These people eventually settle in eastern Indonesia and the Philippines where they begin using shell tools. They also learn (or relearn) the art of edge grinding. They manufacture edge-ground shell and stone tools, and also make fully polished neolithic blades. One of the important tools made by these people was the celt, a groove-less axe. The blade industry is distinguished by the rectangular cross-section of the tools. The shell mound people appear on the South China coast with their shell tools, edge-grinding and roughly polished tools sometime before 5000 BC. They form a culture along the Yangtze River. And they quickly move northward into present-day Shandong. The cultural kit of these people came to include by 5000 BC: clay spindle whorls to make nets, clay net sinkers, disc-shaped earplug ornaments, stepped stone (socketed) adzes, stone hoes, stone knives and long-stemmed polished stone arrow/harpoon heads. They also made Hoabinhian-descended pottery.
The Yangtze and Shandong regions are important. They will become vital nodes in the Nusantao trade network.