Only seen a small clip, I'll see the entire video.
But by all means lets take some one who has only shot pop cans with an airsoft, put them front row center, where they can't move, stuff thier self defense gun, under an overly large T shirt, & when the (Highly Trained) active shooter bursts in they get shot, "So see you can't defend yourself"
If he had taken a seat at the end of the row, & had a proper holster, he might well have been able to have done something, hell they likely tipped off the active shooter as to where the only armed person was sitting.
In part I agree, just having a gun may not allow you to prevail, Training & effective use of the gun will greatly enhance your chances of winning, & as Clint Smith says "You can still lose"
If Jeanie Assam, had been in the parking lot at the church, & been the first person shot , she likely could not have defended the church, that day in december. The attacked are reacting not acting so you do need a few seconds to react.
So if you are the first or second person in line when the shooter bursts in you will likely get shot, if you are 4th or 5th down the line your odds are getting better.
A sheepdog is looking for escape routes, cover etc, & does not paint him or herself into a corner, but 20/20 didn't pick any sheepdogs, I imagine if they had reversed rolls & the firearms Instructor had been the one the shooter would not have gotten far.