Lots of USA expats live permanently in Mexico.
It would make sense for the expats to leave Mexico to be close to better medical facilities.
The Expats fleeing the Mexico covid infestation is very similar to the residents fleeing the NEW YORK covid infestation.
Since air travel is very limited, most of these expats would have to have driven from their Mexican home to the US border of TX, AZ, or CA.
Are you claiming these 'expats' have also forgotten how to speak english?