The answer to this question can be found in Chapter 16. Sino-Tibetan language speakers from northern China conquered people from South China, either forcing their culture on them or driving them into southeast Asia. The people of northern China developed agriculture very soon (around 7500 BC) and thus had a major advantage over hunter-gatherers around them. This made it easier for them to spread their language and their culture, a process which Diamond describes as analogous to the spread of European languages into the New World after contact. China also had geographic advantages that made cultural uniformity easier, such as long east-west rivers and few natural barriers.
the chineses are actually aliens and they fell from the sky.