Earlier quoted context omitted.
This would be a wonderful thesis if it didn't ignore the fact that in the world (and the US in particular) there's a lot of land. So much land that you could fit the entire world's population twice into Texas alone assuming major city densities (assuming 67k /mi^2 and 268000 mi^2 per cursory googling gives some 18B).
Roughly none of that land is freely available. It's also pretty clear that some sort of resource footprint is more interesting than apparent living density. Just food requires more than a few hundred square feet per person and we consume a lot more than food.
You'll have to qualify that statement a little. Here is 5 acres of land in Texas for $1500 [1], found it in about 60 seconds.
[1] https://www.zillow.com/homes/for_sale/TX/land_type/209109538...