excuse me but read the piece again...
they're not talking about 1200 miles of fence,
they are talking about 370 miles.
Besides, it's not 370 miles ~ it's also 500 miles of "vehicle barriers". Plus, there's stuff already built and it's supposed to be "improved", but that mileage, which is considerable, is not part of the mileage estimates.
In any case, some of the border consists of mountains where no new fence will be built (although you'd better believe someone in congress will figure out why billybob's fence company in Mexicali needs to go out there and hang wire off a rockface.)