Your statement is true, but that was clearly not the point being made. Lot's of people coming back to the USA from MX and other countries. As long as they are US citizens or legal residents, we should accept them, and that is what is happening. How well we are screening them is another matter, but that is on CBP/DHS/CDC.
To clarify, the point I wish to make, is the current increases in Covid-19 in the border states has a lot to do with people from outside the United States, particularly Mexico, coming to the U.S. for treatment.
Many of them are U.S. citizens and permanent residents.
Their numbers are being used to clamp down on the activities of people in the U.S., even thought their numbers have nothing to do with conditions in the U.S.