The only discrepancy is that they assume the whole boarder length of ~1900 miles. There are about 800 miles of boarder that are already impassable by natural barriers.
I will disagree with you on the contention that some of the border is impassable. Nothing given to the will and determination to do it is impassable. It may be very tough to do so, but to be more precise, impracticable.
And, the state of art for listening devices and sensors would be the ideal source to monitor those avenues with the human assets posted close by for QRF (Quick Reaction Force) operations.
I know we used them in 1974/5 when I was stationed at the US Army Electronics Proving Ground at Ft. Huachuca. They were actually quite amazing, even in those days. I can only imagine what time and technology has done to enhance that.
“The only discrepancy is that they assume the whole border length... There are about 800 miles of border that are already impassable by natural barriers.”
And there are hundreds of miles of existing man-made barriers of various types.
If you prioritize the high traffic corridors first, you can throttle the flow of migrants big time with the first few hundred miles (around the cities and major roads).
After a few years of careful monitoring and policing, patching one access route after another, it would not be feasible to have mass movements. As it became harder, less would try.
If law enforcement was toughened up, and economic incentives were reduced, they would have equal or greater impacts on the numbers.