I'm not the person who wrote that, but I think "strip mined" means that "MAGA" views foreigners (legal and illegal) as taking advantage of the US and depleting its resources.

FWIW, I think there's a legitimate complaint that illegal immigration depletes our resources (but there are also positives). I have a friend who teaches at a high-school with a large population of 'undocumented' students and he's constantly railing about how they unfairly use school resources to the detriment of the other students.

I think it's a much less legitimate complaint about legal immigration, visas, etc.