I am willing to bet that there are a lot of variables that math doesn't calculate for, such as mass farming inefficiencies that can be accommodated for by small

I am willing to bet that there are a lot of variables that math doesn't calculate for, such as mass farming inefficiencies that can be accommodated for by small-family farming, monoculture crop dominance, space for heavy machinery and food storage, and what agriculture we're talking about (as, for instance, the corn grown for conversion into ethanol or high fructose corn syrup isn't always the same kind of corn grown for human consumption, or hay and other grains grown to feed cattle, sheep, pigs, and chickens). There are also extra inefficiencies due to greenhouses, orchards and the like that take up a lot of space for little resultant plant matter, and I don't know if you checked but if that figure accounts for cattle grazing that in addition to needing land for food to be grown for them also requires land for them to occupy and live in until they are slaughtered. I'm not saying you're wrong, just more that your figure puts a minimal upper bound on the amount of land needed to feed a human rather than placing a minimal lower bound on it.