In Chapter 1, Diamond is essentially saying that no continent truly had a head start in 11,000 BCE. 11,000 BCE is just before the time when agriculture is discovered. Before this, no continent had any real sort of an advantage in Diamond's mind.
Diamond says that all the continents could have been said to have advantages at this time before agriculture began. He says, for example, that you could predict that Africa was the best off because it had been populated by human beings for longer than any other continent had. However, he says, you could make just about as good a case for any of the other inhabited continents.
The point of this is that all the continents were essentially equal in 11,000 BCE. It was only the development of agriculture that caused them to diverge in the years following that date.