Interesting results, wrong conclusions. Plus they didn't separate cause and effect. "He demonstrated tht [sic] the number of years since a society made the transition from hunting and gathering to agriculture is highly correlated to the level of income in the country today. That is, the earlier the agriculture development, the higher the income." Or exactly the other way around - the better the person is at producing…
It might have more to do with when they first had to switch to agriculture. Paleolithic hunter-gatherers had a better lifestyle and better nutrition than early farmers, so why start farming if you don't have to? Farming is tedious, backbreaking labor which gives you grass seed to eat, while hunting is something men do for fun even when they don't have to, and gives you meat.
I don't really see the connection between farming and intelligence. Which would you rather trust a stupid person to do: plow a field or kill a lion with a spear?
My first thought was that adapting to the tedious, persistent labor of farm life would lead to useful traits in a modern economy, unlike hunting, which is more like a leisure activity.