I don't know the details of the NRA instructor, but the other guy said it was that his shirt was not fully tucked and put enough drag on the trigger going into the holster to fire it.
The only striker fired gun I have is a Springfield XD, I religiously push it into the holster with my thumb at the back of the slide so the grip safety is not engaged. I like the extra layer of safety.
Call me inexperienced or untrained but I don't have a round chambered unless I'm actively defending myself or stalking a trophy bull tweaker. I don't carry often so maybe it's a familiarity comfort thing but I figure I can get a round in the pipe pretty quick and the sound likely is an attitude changer which works all the same for me. I don't have a gun fetish but see their utility and necessity. Being in brush and bullshit with a round in the pipe just don't feel right. For that reason a revolver is my brush gun of choice and I can treat it like garbage and it still works. With a 9mm or similar I go by the Israeli philosophy and am familiar enough with my weapon to pretty instantly get it hot. I'm not judging anyone for a different protocol and would like to hear from anyone who has more knowledge than myself if I'm doing things incorrectly.