Obviously no one is suggesting turning the US into a tomato monoculture, but tomatoes seem like a easy, if not necessarily accurate, proxy for the sort of fresh vegetable passed diets that some people advocate as a solution for the entire nation. If the did the same calculation for lettuce, the numbers would likely be much worse. If I did it for sweet potatoes, I’m guessing they would be substantially better.
There is a very useful resource on growing tomatoes made available by the Iowa State extension service. They estimate yields of 12,000-16,000 pounds of tomato per acre. A pound of tomatoes contains 86 calories.