The answer to this can be found on pages 89 and 90 in the paperback version of the book. There, Diamond tells us that having stored food surpluses allowed farmer societies to have professional soldiers. This made them more powerful than hunter-gatherer societies.
On p. 90, Diamond explains to us that it is very hard to have any sort of specialists in a hunter-gatherer society. He says that all people in such a society who are able-bodied have to spend most of their time getting food. This means that there is no chance to have people who truly specialize in anything outside of getting food. You cannot have people who spend all their time being priests or scribes or anything like that.
This makes hunter-gatherer societies weaker. Diamond says that stored food surpluses can feed some people who specialize in fighting. It can also feed people who specialize in making metal weapons or those who specialize as priests who “provide religious justification for wars of conquest.” Because the food can feed all of these people, the farming society becomes militarily more powerful than the hunter-gatherer society.