The answer to this is found in Chapter 2. Diamond uses the Polynesian islands to determine whether environment or ethnicity/culture determines how advanced a society becomes. He points out that Polynesians (all the same ethnicity to begin with) spread out to many islands (like New Zealand and the Chatham Islands) that had very different environments. He notes that the people who lived on big islands with lots of resources (like New Zealand) became more powerful and had more complex societies while those who lived on islands with few resources (like the Chathams) became weaker and had less complex societies.
Thus, these islands are an experiment because they allow us to see what happens when one ethnic/cultural group ends up in many different environments.