One of the points made in the video is that it's much easier to conquer and rule people who grow grains than people who grow root crops. Grains have to be harvested in a timely fashion-- the granaries can be looted, the fields can be burnt. If your soldiers have to dig up the potatoes, it just isn't worth it.
Yes, it's easier to loot people who grow grains than roots, but I don't think that's so relevant to taxation by a stationary bandit.
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