Yeah, actually he could. Less than a 15 degree angle of declination was all that was needed. With windows broken out down to the floor his muzzle could have been 3 feet back from the edge and he could still have hit the concert goers. Do the math.
15 degrees? How far was the concert, and how high was the shooter?
Three feet back from the edge would mean he could see the horizon, and that is it.