The Mythbusters did an episode on free falling bullets. IIRC, they concluded that a bullet shot straight up would not be lethal on a free fall trajectory, but, I believe they did conclude that a bullet returning to the earth in a normal trajectory could be lethal. The primary difference being a free fall bullet would be tumbling and have less velocity where as a bullet returning on trajectory would likely still be spinning and have greater velocity.
Still, in this case, I suspect we may hear more to this story. It seems too improbable to me for a black powder projectile, even a sabot, to carry this far. Suppose the wind would need factored in, but still....
this is a black powder firearm. If it is a brown bess firing a 3/4” lead ball, it is probably lethal in freefall. If it is a ten gauge black powder shotgun firing a .775” diameter lead ball, it is probably lethal in freefall.
NRA did a much more exhaustive study during WWII. Bullets fired straight up,, something like 80% tumble and fall presenting very little hazard. The other 20% maintain their spin and return to earth base first,, still spinning,, and are deadly.
And anything fired from around 45 degrees angle,, returns to earth at etreme range, very close to muzzle velocity.
A parabollic bullet trajectory slows to near terminal velocity, not the direct fire one which obviously still has kinetics despite having less potential energy and power developed.