Well, this should be true, but subsidies go to the luxury foods, mostly animal products, not the most efficient nutrient to cost ratio foods with minimal externalities.
Humans do not eat corn, wheat or soy in coming out of current production levels.
First of all, national security requires having food on hand that people actually want to eat. You know what happens when you go to war and suddenly the only food left is corn? Riots against the war and a massive black market.
Second of all, subsidizing foods that people don't want to eat is inefficient since people, amazingly, won't eat it. So now you have a lot of food that you need to dispose off. You can't just make less of it because then it won't be there in case of an emergency. So you're almost literally throwing money into the dumpster.
Subsidies are made for luxury foods. Luxury foods are not kept in storage houses losing value, luxury foods are sold fresh.
If you grew beans, the best protein, calorie per externality (smallest energy required to grow), then you could store it, subsidise it and be really safe with that choice.
There is no national security in growing energy expensive foods. If you can't input the energy required to grow it, you're going to need to subsidise it even more in times of crisis. It's just plain simple stupid strategy and is not robust, not antifragile, it's fragile ridiculous strategy.
Luxury foods are subsidised because of insane lobbying. That's the only reason. No bean farmer is going to ask for subsidies given that his current demand is no where near beef.
Farmer that is working for Tyson Foods is going to ask for subsidies because the business model of Tyson Foods is made just to extract more money from government.
National security isn't just protecting from outside, it's also protecting from within. Stabilizing society via bread and circus is a valid realpolitik strategy, and what better way to do it than boosting the production of food to post-scarcity levels
Humans do not eat corn, wheat or soy in coming out of current production levels.