Yes, it's probably true that the US has the agricultural capacity and technical expertise to provide enough food output to supply the world's population. However, absent a business model that supports that level of output, we can't sustain it.
Tractor fuel, electricity, equipment repairs, farm wages, fertilizer, cartage... all these things cost money. Farmers and agribusinesses do not charge money for the food they grow because they're filthy capitalists; they charge money for the food they grow because they have to meet expenses (and because they're filthy capitalists too, but only after the expenses are paid).
Unless the world can pay market prices for the food we produce, we cannot feed the world. It's not a matter of being greedy... it's a matter of making enough return on our investment to buy seed wheat to plant in the spring, and fix the combine that went tits-up late in the summer, and truck in another semi-load of No. 2 Diesel to run in the tractors, and pay the farmhands Darrel and Lloyd and Earl for their time. If you can't do those things, the bank auctions it all off and you've got no farm.
Having a practical business model is a necessity, because without it you do not have a going concern. Making a business self-sustaining can be as challenging as making a fusion reaction self-sustaining.
And about that... the way fusion power plays out in the marketplace will depend heavily on what mode of fusion gets its breakthrough first.