In this book Diamond never actually defines "laypeople." However, he does make clear what he means by this term. Basically, he is saying that a layperson is a person who has some sort of a stereotyped or perfunctory understanding of history that is not really based on clear thought.
As an example of this, we can look at Chapter 13, page 241 in the paperback edition of the book. There, Diamond talks about how Eurasian societies had so much more technology than the people of any other society. He notes that many other people had the mineral deposits necessary to create things like iron tools but never did. Diamond says that laypeople take that as evidence that people in other parts of the world are inferior to Eurasians "in inventiveness and intelligence."
From this, we can see that Diamond uses the term "laypeople" to refer to people who have only a surface understanding of history.