I realize the trainee in the video is some kind of LE, and that parts of the video was cut and pasted so we don't get the whole thing.
Having said that, if I were in his position as non LE, but armed, I think I would be inclined to not do anything except seek cover, escape if possible, and observe. I'm not totally happy with that answer, however, I'm not store security, nor am I responsible for the clerk's safety. I am responsible for my safety. If the weapon comes around in my direction, or I am being robbed, that changes things. I don't think that I would immediately jump to shouting "drop your weapon" and start shooting.
Hindsight is a wonderful thing. I can see the benefit of this kind of training for sure. In the training portion they did mention something about looking around after the shot, maybe that should have been a clue, but then again he probably got a lot of advice we didn't see so it's most likely unfair to say he missed that.
I'm open to different opinions on my position.....