A friend of mine wrote a scholarly article on range estimation and found that the degree of error increases as range increases. Average people can estimate range out to 100 yds pretty well (usually within 10%). Experienced hunters tend to do slightly better, usually maintaining the same margin of error out to 150 yds or so. Past 200 yds, range estimation drops off significantly.
Either the range is incorrect, the kid was incredibly lucky or someone is blowing smoke. If it did happen, it was certainly unethical. I personally won't take a shot over 400 yds at a game animal, ever. I won't go over 300 without a dead-solid rest with a gun and load that I have used extensively and having adequate power to do the job.
As an aside, red stag are taken with a .243 as a matter of policy but the ranges are quite close.