I'm not sure doubt of the shot itself prompted the Mythbuster's test. Rather it was a question of terminal ballistics. Considering they had no clue as to the type of scope used, apparently, I can see where questions would arise. I.e., did the bullet itself cause the wounding or was it secondary projectiles? Did the bullet exit the scope tube before piercing the ocular glass or did it break out of the side of the tube? Did the bullet exit intact or in fragments, and, if so, were the fragments capable of a fatal wound? Minutiea, but interesting in a grisly (or forensics) sort of way.
BTW, now that the distance is known, who's going to volunteer to round up a bunch of Sovietscopes and try again? This time get all the parameters correct and see what the likely outcomes were. No doubt the sniper had a bad day, but just HOW did his day end? I reckon there could be several end results that vary on very small differences in the impact. Too bad there's no real practical value to the experiment.