One of Diamond's major arguments in this book is that long east-west axes allow crops to diffuse -- to move from place to place.  This is because places on one east-west axis tend to have similar climates and can support similar crops.  Along with the crops, technology and culture can diffuse as well.  This helped China "Sinify."

Because China had a long east-west axis, it was easy for one culture to come to dominate China.  If China had been broken up by rivers and mountains the way Europe was, diffusion would have been much harder.  Instead of having a unified Chinese culture and state, there might have been a situation more like that of Europe where there were many small countries.

So, because China had this type of axis, one culture was able to diffuse and make China a fairly homogeneous place -- to make China Chinese as Diamond says.