The answer to this can be found in Chapter 13 of the book, beginning on page 243 in the paperback edition.
Diamond says that most people believe that necessity is the mother of invention. They say that inventors see needs and invent devices to fill those needs. Thus, invention becomes a virtuous and even heroic act and societies that have many inventors are seen as superior to those without.
But Diamond argues that Edison’s example (and many others) disproves this view of inventions. He says that Edison published a list of ten uses for the phonograph. He did not put music high on that list. He said that he did not think it had any commercial value. Clearly, he did not invent it for the purpose to which it was most commonly put. Diamond says that
Only after about 20 years did Edison reluctantly concede that the main use of his phonograph was to record and play music.
This is important to Diamond’s argument because it helps disprove the idea that societies with inventors are superior to those without them. Diamond argues that societies with more people and a longer history of being sedentary are more likely to invent things. It is numbers and time, not cultural “fitness” that makes inventors arise.