They did mention that if shot at enough of an angle, the bullet COULD POTENTIALLY retain enough of its initial velocity along the horizontal plane to kill someone if it stuck in the right spot.
If they had gotten in an imaginative physicist, they could even have written an equation for it.
There are two components to a bullet's flight, the forward (F) and the downward (D). So long as the bullet remains in a stable flight path, the D force is insufficient to kill someone. If it leaves a stable flight path, tumbling ensues, and the D force is reduced. Only in the very unlikely occurrence (for most bullets) of a tumble at high altitude turning into a stable D path (with F reduced to zero) can the D vector possibly kill.
Any time D is sufficiently large compared to F, you'll also get tumbling. That happens in two cases: in a mostly horizontal flight, near the end of its path (if it hadn't hit anything first, which is unlikely), or in a mostly vertical path, as the bullet begins to fall. So a bullet that's going to be potentially lethal can't have too much of a vertical path. OTOH, though, it can't have too much of a horizontal path, either, or it will hit something nearby and can't properly be called "stray".
The angle will depend on the bullet design; some tumble easily, others are intended to remain stable over a long distance. But in general, a bullet fired at over sixty degrees from the horizontal isn't going to kill anyone. One fired at less than thirty degrees isn't likely to, either; even in a small town that trajectory is flat enough that it's almost certainly going to hit something before it hits a person (unless someone is so foolish as to fire right down a street, or toward a park).
But again I have to ask, haven't these people heard of blanks?
And why isn't there an ammunition company out there making celebratory rounds? Imagine loading up blanks that fire red, white, and blue smoke...!