I think that people tend to believe that farming is clearly superior to hunting and gathering. All civilized people (so pretty much everyone in the world today) live off things that are produced by agriculture and so we think that this is the way that is obviously the best. I think that Diamond is challenging this assumption.
Diamond points out that there is nothing that is inherently inferior about hunting and gathering. He says that it is not a bad way to live. In fact, he says, it is in many ways better than a settled, agricultural way of life.
So Diamond is challenging the idea that people consciously and deliberately changed to agriculture because it is a better way of life.