actually they have things backward.
Poor people rely on root crops to fill their stomach. And potatoes were not in Europe or Asia until 1700 or so: in Eastern Europe they had the advantage of being hard for armies to steal when they traveled through.
Remember when Scarlett O'Hara ate raw turnips, because they were the only thing left behind by those damn Yankees?
These maps show a clear correlation between crop choice and political complexity. Societies that grew grain tended to have more hierarchical political systems — empires, even — like the rice- and wheat-cultivating kingdoms of ancient India. Tuber crops were associated with smaller, more local political units.
yes, but that was often because these people lived in marginal lands.