I have read the same thing on elevation.
Btw it wouldn't work if there was NO elevation. Convergence would be futile. Imagine the slow 30mm (Mk103) at 650 yards, the shells travel 5-6 yards below the target if they came out parallel to the sightline. For .50 that would be roughly 3 yards.
So you would hit centermass at vertical axis, but fire entirely underneath the enemy plane most of the time, even with 50cal.