It might be BS, but it could be true.
Records (of any sort) don't need to be easy, they merely need to be possible. An average hunter might be exceedingly unlikely to hit a deer at 600y while standing, but since this is not impossible it can't be ruled out simply because it is highly unlikely.
According to the story, he got the correct elevation for the first shot. That's not how I would have bet in the circumstances, but there's nothing saying that that can't happen.
He also got the wind correct for the first shot. This is certainly possible with the correct combination of "good" and "lucky", in fact given that the story said that conditions were ideal, it's not terribly surprising.
So he gets a first-shot hit. While the way to bet would be a near miss for his spotter to correct off of, there's nothing saying that a first round hit is impossible.
When his first shot landed, he would definitely be reloaded and back on the target (for several seconds in fact) waiting for a correction from his spotter. Since it was a hit, which he almost certainly would have seen, he wouldn't even have to wait for his spotter's correction. He likely aimed at target #2 and broke his second shot less than two seconds after his first bullet arrived.
The incoming bullet was subsonic. So Bad Guy #2 would not have heard a sonic boom, there would only be the impact noise. If anything else was going on at the time (noise, action), it's quite conceivable that he would not immediately realize that Bad Guy #1 had been hit.
Since (as it turned out) the shooter was dialed in for shot #1, the chance of shot #2 also being a hit was pretty good. So 6-7s after the first shot arrived, the second bullet would be there, and Bad Guy #2 was hit.