That's part of it, for sure (it comes with genociding off the majority of the indigenous population and developing as a settler-state instead of natural demographic growth). The US' land could easily sustain a population the size of China's or India's.

Alongside that, the mechanisms of this agricultural imperialism are such that it's not some "complete support of other countries' agriculture"- rather, it's implementing the same kinds of agricultural dependencies that were and remain endemic to, say, the Caribbean, where local agriculture would be focused on cash crop plantations which could not sustain domestic needs.

There's also a space for other countries' agriculture (of staple crops, etc) within this model- but it is under western ownership. The process generally goes as such- agricultural subsidies and state-led or locally owned agriculture is disincentivized or worse, and meanwhile the markets are forced open- thus, heavily subsidized western agriculture flows in and destroys local farmers. Then afterwards the US swoops in for the fire sale..