I guess I'd have to see what "extreme accuracy" means. The. 50 BMG round in and of itself is not all that accurate (in comparison with other available calibers). It has the range, no doubt, but not repeatable accuracy.
1 mile is 1760 yards. 1 MOA would be +/- 17.6 inches, or approximately the width of a man. Remove all atmospheric variables and there is still no way in hell that weapon system is capable of 1 MOA.
Not to take anything away from the shooter: he still had to get the bullet in the neighborhood in order to connect.
Since they had a scope adapted to the gun, I'm wondering if they went to the trouble of using special loads. I disagree regarding 50 BMG inherent accuracy, especially for the AMAX and similar projectiles with a BC of 1.05 or more. Nothing else comes close to that BC. The Hornady AMAX 750 is easily capable of sub-MOA in the right gun, as are some of the high BC specialty turned solid projectiles.
That M2 would not be the right gun, LOL. The standard ammo isn't that great either. In a heavy barrel, single-shot target rifle, your typical 50 BMG surplus rounds will do ~1.5 MOA at best and 2 MOA is probably a reasonable assumed accuracy. Not terrible, but not sniper worthy.