The argument depends on the differences between how grains and tubers are grown. Crops like wheat are harvested once or twice a year, yielding piles of small, dry grains. These can be stored for long periods of time and are easily transported — or stolen.
Root crops, on the other hand, don’t store well at all. They’re heavy, full of water, and rot quickly once taken out of the ground. Yuca, for instance, grows year-round and in ancient times, people only dug it up right before it was eaten. This provided some protection against theft in ancient times.
interesting proposition that grains advance a people but tubers hold back....... but they seem to miss what I think would be the obvious reason....
in describing grains how it can be stored and transported they’re describing something that’s inherently economically “liquid” like cash.... versus the the tubers
To have of your crop “liquid” with condensed value for trade has driving a lot of things
consider the Whiskey Rebellion which was the farmers then took their grain and concentrated the value into something simpler the transport and store which was alcohol... and that drove government taxation
One of the big problems imho with their whole premise is, the cultures which rely on tubers for much of their food supply, while they are settled in one place, have for the most part never developed writing, or those who have, are yet undeciphered for the most part -- IOW, there's a lack of data regarding how long they've lived there, whom they'd displaced/annihilated, and from whence they'd come.
One could as easily argue that the grain-growing civs are mostly riverine (rather than sprouting on islands or at high altitudes like Peru), which mitigates in favor of grains vs tuber crops, and the culture got started in the same spot much, much earlier, and had time to develop, for example, writing (some writing systems seem to have arisen from a need to keep track of property boundaries, ownership, etc, and then taxation).
Cultures relying on tubers never had either necessity (staying in one place, or property boundaries, as the nutritional output is more dense, and less land is required). If anything tuber storage is much simpler, it just remains in the ground until mealtime today or next quarter. Grain requires more land, and more time for that matter (if one could live on radishes, those go from seed to snack in 30 days; right now I've got a field ouut back of still-immature winter wheat, planted late last fall, and probably to be harvested after nearly a year after planting), and that land has to be kept under control.
IOW, I think the researcher may have benefitted from some conversations with farmers.