It wasn’t measured afterwards. It was measured prior to the shot. That is how they knew exactly how high to aim. The sniper works with a spotter positioned right next to him. The spotter calls out the distance, temperature, humidity, wind speed and direction and makes the computations required for how to adjust the scope. The spotter verifies the kill or calls for another shot. Sometimes it can take 5-7 shots to hit the target. The bullet gets there way before the sound does.
The sniper can’t even see a person at that distance.
Oh! Well, that does make the documentation of calculations for world record purposes a lot more palatable. I appreciate the explanation. If a bullet arrives before the sound, I guess one wouldn't hear it? Scary.