In order to answer this, let us think about how a scientific experiment is carried out. Let us say you have chemicals A, B, and C. You want to know which of A or B will have a greater reaction if mixed with C. All you have to do is add A to C in one test tube and B to C in another. It is simple.
In studying history, it is not so easy. Diamond wants to look at culture and the physical environment and determine which one has more of an impact on how powerful a society becomes. The problem is that he cannot go into a lab and give two societies identical environments but different cultures, or identical cultures but different environments. Therefore, Diamond has to hope that there are “natural experiments” that he can look at to see whether culture or the environment has more to do with a society’s power.
The Polynesians give Diamond that natural experiment. The Polynesians all came from the same original culture but spread out across the Pacific to places with different environments. For example, the people who came to be called the Maori settled in New Zealand, where the environment was good for agriculture. By contrast, the Moriori settled in the Chatham Islands, which cannot really sustain agriculture. Thus, you had a natural experiment; you had people with the same culture settling in places with very different environments. If culture is what mattered, the Maori and Moriori should have been equally powerful. Instead, the Maori were much more powerful, thus proving (or strongly implying) that environment is much more important than culture.
Thus, the Maori and Moriori were a good natural experiment because they were an example of people with identical cultures being placed in different environments.