Significantly, this was written before the discovery of perchlorates in Martian soil, at levels that are toxic to humans. At a minimum, this means that human settlement will only be possible after a significant long-term terraforming process, not before.
> "If you have one country that can grow wheat and cotton better than their northern neighbor who can just grow wheat with lesser yield per acre..."
You're describing absolute advantage, not comparative advantage. Even if country A is worse at growing wheat than country B, it can still gain by specializing in exporting wheat to country B... so long as it's even worse at making other goods.