That assumes that once anyone enters the US, they will, on average, produce the same amount of CO2 emissions as present US residents. That seemed dubious to me... so turned up this: https://cis.org/Immigration-United-States-and-WorldWide-Gree...

> The estimated CO2 emissions of the average immigrant (legal or illegal) in the United States are 18 percent less than those of the average native-born American.

> However, immigrants in the United States produce an estimated four times more CO2 in the United States as they would have in their countries of origin.

So I suppose the overall logic still holds, just slightly adjusts the math.