I would vote yes hes fibbing. Its not impossible but with that bullet you have to aim about 5-6ft higher then the deer and unless you practice that it would be a very hard shot.
With a 140 gr slug sighted in 3.9" high at 100 yards will give you a 300 yard zero. You would have to aim roughly 30" high at 500 yards. A far cry from 5-6 feet. A 150 gr slug would have a little more drop, but not that much. It could certainly be done if you practiced and knew the exact yardage. I wouldn't try it, but it can be done.