You can find the answer to this question by reading in Chapter 2. There, we can see that Diamond calls the islands a natural experiment because they allow him to test the relative importance of culture and environment on societies.
The islands of Polynesia had very different environments but were inhabited by people who all came from the same original culture. If culture were the main factor that affects how societies end up, all Polynesian societies should have been essentially the same. If environment is the main factor that affects how societies end up, the societies should have been very different from one another.
Since Polynesia was made up of different environments all settled by people of the same culture, it was a perfect "natural experiment" for determining whether environment is more important than culture in determining what a society will be like.