Firestarter,
Doing it slowly without allowing the bolt to fully seat the round, one time, to test a bullet's contact with the feed ramp isn't terribly unsafe if done pointing the gun down range (in a safe direction) with one round in the mag and finger off trigger. What's the worst thing that can happen? You shot one round at a target?! Whoop de doo!
The whole point of The example was you don't have to remove the magazine to test a single round on whether the firing pin will for certain ignite the round, as you suggested. You can cycle it and test it with the mag.
Warning to those reading this: if the bolt rests however gently on the round, it may go off at any time, even immediately (it would depend on how strong the main spring is, how sharp the firing pin is, and how strong the primer metal resists the pin under pressure). The point being that the safety ENGAGED on a live round with the bolt closed won't make it safe. It was just an illustration to explain mechanics of this aspect of the safety.
I didn't and don't suggest that anyone should attempt that, unlike fire marshal bob who vehemently defends and in fact advocates using gasoline as a cleaner.