How does the fact that the Maori defeated the Moriori support Diamond's argument in Guns, Germs, and Steel?
The answer to this can be found in Chapter 2.
Diamond's main argument is that geography, not race or culture, is what makes some peoples powerful and others weak. The case of the Maori and the Moriori demonstrate this. The Moriori on the Chatham Islands had come there about 1,000 years before from New Zealand. This meant that they were racially the same people as the Maori and had started out with the same culture. But the Chathams were no good for agriculture and so the Moriori became hunter gatherers. Meanwhile, the Maori remained on New Zealand and farmed. This allowed them to have a society that could create more weapons and have more political organization. These things allowed them to defeat the Moriori.
Since the two groups were originally the same in terms of race and culture, the only thing that caused the Maori to win (Diamond argues) is the geography of the places they ended up living.