Hmm, much debate ...
I think we have to take the kill as verified, given the clear statement that a second OP saw the whole thing. They won't have *easily* been fooled by a hit from shorter range (if indeed there were any friendlies closer to the target).
And yes, if it was the first shot, that's an absolute phenomenon and I expect the sniper himself to accept he had some amazing luck—but let's be clear, even with an exquisitely built, selected and polished 50 cal round in a supremely well-engineered rifle with pristinely accurate optics and the range lasered to the centimetre, it would still take only a brief thermal out of some intervening wadi, anywhere 'twixt muzzle and target, to make a miss that wouldn't even ruffle the bugger's hair. My guess is that having got the range and windage as perfect as possible, our sniper friend expended at least a handful of rounds before floating one onto our bad boy. I don't detract from his skill in the slightest—hitting a double decker bus at that distance is impressive with anything smaller than a 20mm autocannon—but the intervening variables of air and even gravity are profoundly perturbative at those distances, even with a chunky 50 cal slug. (I'm not familiar with the "Tac-50" but I guess it's fundamentally similar to the Barrett).
By the bye, I'd be very interested to know if that shot is even possible with a smaller round, like the 7.62 we used to have in our slurs. I'm guessing even a bench-viced AI loading 7.62 wouldn't hit a man-sized target at 3.5 klicks out of fifty rounds: ballistic coefficient simply too small?
PS: This is a strange thought, but I just found myself wondering: what perimeter do the Secret Service enforce for the presidential protection detail, when he's exposed outside? Will it increase now?