See the original comment here.
Greer is looking at agricultural societies. The farmers there could just pick up and join another agricultural society if their society collapsed, even if the existing society collapsed.
I don’t see a way we could build up enough of a network of knowledgeable people to build an agricultural society. We would need to do it without electricity, oil or other fuel sources. We don’t know how to go back that far any more. We would need to figure out how to do everything with local materials, everywhere. We would need to build local defenses of cities–moats? All of the attempts at working with wind and solar PV are a waste of time, in this regard. We would after a while even have to do without metal tools–they are just too demanding of wood. - Gail Tverberg