Watch the slow motion section again. You can see the exact moment his finger leaves the trigger guard and pulls the trigger, it happens well after the gun clears his holster. You can blame the holster if you'd like (even though he doesn't). I blame poor trigger discipline. The bottom line is your finger shouldnt be on the trigger! Its NOT easy to "slap" the trigger on the way out of the Serpa holster. I dont know about you but all of my pistols have a trigger guard, if you have ever used a Serpa you would know that you pretty much have to keep your finger straight to release the button. A straight finger doesnt slap anything, it rests on the trigger guard after the gun is drawn. A curved finger that might accidentally land on the trigger wouldnt be able to reach the button to release the pistol. This is just based on my observations and my fingers on a variety of my Serpa holsters with my pistols, so YMMV.
What really surprised me overall is that he took a .45 round at point blank range and didn't immediately keel over from hydrostatic shock and failing internal organs. Oh, right, it was a glancing shot
I have used SERPAs several times and it may just be the way we are different in build. But, I'm 6'4" and have big hands and my finger has most definitely slapped the trigger coming out of the holster. The way I discovered that this was not an unusual problem is because I was concerned about it in the first place and did some research. In a tactical situation the last thing you need to be worrying about is how much pressure you put on the release or getting your finger to hit the guard rather than the trigger when you draw. As the man said, he has done this draw thousands of times without incident, but then it happened. It's one of those things that is just a matter of time before it happens because the basic design is flawed.