The Polynesian islands are an experiment of history because you have people from the same culture and ethnicity who go out and colonize a whole bunch of islands. If culture or race is what causes some people to become "civilized" and powerful, then all of these islands ought to have ended up at the same level of civilization and power. If they do not end up that way, it probably indicates that something other than race or culture is responsible for the outcome. So it's an experiment -- we have the same kind of people colonizing very different places -- what will happen?
What Diamond concludes is that the environments of the island determine whether food production will develop. When food production develops, those islands' societies become stronger and more developed. Diamond says that culture doesn't matter -- societal development is determined by how productive their environment is.