Not to put words in his mouth, but I believe he's saying after the projectile has passed it's apex, it might regain some velocity because of gravity. This of course would be assuming a fairly significant launch angle.
But, you're right - as soon as it leaves the barrel, it begins to slow down at a predictable rate.
That is how I interpret the story as written. Kind of a Saddam rifle shot from the podium.
“it might regain some velocity because of gravity”
Not unless the bullet was thrown by hand, and slowly at that. I know what he was thinking, but that would only be true in a vacuum.
I’m guessing AK47 fired directly at the roof. Up to about 500 meters, perhaps a bit more, it might still have the velocity to penetrate a roof and deflect downward with enough force to kill. The AK bullet isn’t the fastest or most aerodynamic bullet, but it is quite common in the inner city.