I remember when we were confident the U-2 could not be shot down .
20,000 feet .. ain’t much of a reach.
Indeed, consider this statement...
Such airships would serve as surveillance stations at 20,000 feet (6,096 meters) above sea level and could stay on watch for as long as three weeks at a time.
Then, realize that much of Afghanistan lies in the range of 8-12,000 feet above sea level. That leaves 8-12,000 feet of air under the airship.
As I recall, the max. vertical range of a Stinger is around 12,000 ft. I imagine the LEMV won't have much of an infra-red signature, but the margin-for-error doesn't amount to much.