Diamond does not actually tell us what he means, but I do not think that he is talking about the ability of Aboriginal Australians to survive in intense conditions. After all, there are many cultures (take the Inuit, for example) who can survive in intense conditions. Instead, Diamond calls Australian societies "distinctive" because they are so different from other human societies in technological terms.
On page 297, Diamond tells us that the "most salient feature" of Australian societies was their (in Westerners' eyes) "backwardness." He points out that Australia is the only place in the world where there was no farming, no herding, no metal and no bows and arrows. They did not even have villages or chiefdoms (which Diamond identifies as the second least advanced social system). Because of these things, Native Australian societies were different from any other modern human societies.
I believe that this is what Diamond means when he calls these the "most distinctive" of human societies.