I carry SA revolvers and I NEVER cock one to index it. But then the practice of a empty chamber had a lot to do with rimfire ammunition that was used during early black powder cartridge, and with cap and ball. The Remington 1858 was designed to have the hammer rest between the cylinder in notches. Center fire cartridges are safe with the firing pin resting between the cartridge heads, nowhere near the primer. If he had made a mistake with indexing it could have resulted in a horrible disaster. A lot of NDs happen from improper indexing.
I had seen it several times shooting in SASS where someone indexed wrong, and dropped a hammer on a empty chamber. I shot cap and ball in SASS so indexing was easy. While manning the loading table if anybody did what he did they would have been disqualified. If he is going to index he should use a dowel rod cut to even with the barrel on a empty chamber. To check for proper indexing the dowel will not fully insert, then there is a cartridge under the hammer.
BTW you can just load five rounds without skipping one, and just lower the hammer from half cock and it will result in empty chamber under the hammer.