Along with the earlier 26 and 150 miles of the earlier phases, the total is about 1,250 miles of strong barrier, out of a 1,950 mile total border.
I assume they mean that the remaining 700 miles is so rugged and remote, that it delays/deters even more than a thirty foot wall. Nothing is impenetrable, but 1,000 or 1,250 miles of strong barrier strategically positioned, manned by a strong Border Patrol equipped with the right equipment and technology, can effectively control the border (no remaining safe or easy routes).
For example, this stretch of the Rio Grande River border in Big Bend, is more challenging than a 30 foot wall:
