It still means that the US could probably rely on domestic production if it was forced to do so. In a war, no other country could leverage their position by threatening to stop food exports to the US. It would surely result in less choice, but wouldn't really hurt the US.
My expectation is that it would far more expensive, raising food prices and doing significant harm to the economy. Also imagine the massive costs of shifting production and the gap in food availability during the shift. But I agree that the U.S. wouldn't starve.
I'm not sure about that... a large part of subsidies is to compete in foreign markets. A LOT of food is produced in the U.S. and the majority of trade is so we can get the likes of strawberries year round. A lot of the food staples like grains, beans, nuts, cereals, etc are domestic. Even a lot of the soy and corn. That doesn't mention that we do supply our own meat the vast majority of the time.
I think what would harm us more in total isolation would be the lack of more scarce resources (those used in electronics, batteries etc). We have a lot of materials here, but there are others where China and Africa trade are needed.
I understand the speculation, but if one learns about how economics, trade and the U.S. economy (and all other economies) really work, these hypotheses are clearly wrong.
