given the rainbow trajectory & generally quite short range of muzzleloading rifles (i have 3 of them) i find this report "questionable" at best & "highly unlikely" under any circumstance.
while i fully understand that "falling bullets" CAN & DO kill, i've seen no evidence that a typical ML bullet would travel a mile under any "upward angle of fire" whatever.
fwiw, when i was stationed in LA during the Olympic Games in 1983-84, we had two people in the metropolitan area KILLED & several people injured by "falling bullets" fired from centerfire rifles, which were shot "up into the air" to "celebrate the holidays". - this foolish/common practice seems to be an "LA thing".
(one of the "injured" was a civilian security guard, who worked for our military headquarters command, who was "making his rounds" about 2300 on NYE & was struck by a .30 caliber bullet in the left shoulder, near the collarbone. - thankfully, he was NOT badly injured, was treated at a local ER & returned to work 2 days later.)
i don't think that i have to say this to our forum members, BUT to any "lurkers", "shooting up into the air" for ANY reason is a REALLY STUPID & dangerous thing to do.