In order to understand why this is so, we must first understand what Diamond’s main argument is. In this book, Diamond’s main thesis is that the societies that became powerful in the time up through the European age of exploration did so because of geographical luck. Diamond argues that societies did not succeed because they were in any way superior to the societies that did not prosper. Instead, the successful societies were simply luckier. The Maori victory confirms this argument.
The Maori and the Moriori were from the same culture. They were both Polynesian peoples. However, they ended up colonizing very different environments. The Maori colonized New Zealand, which was able to support agriculture. The Moriori colonized the Chatham Islands which could not support agriculture. If culture or ethnicity determines who succeeds, then the Maori should have had no advantage over the Moriori. However, if the Maori were superior, it shows that their superiority came from their environment, not from their race or ethnicity, just as Diamond argues.
In actuality, the Maori easily defeated the Moriori. This tends to prove that Diamond is right and environment and geography are more important than culture or ethnicity.