Do you think a hive mind emerges when armed people are shopping, where they all instantly know how to communicate with each other to pin down an active threat.
The clear distinction between the military tactic you're describing and random armed people shopping should be obvious. You can't really strategize without communication. And if someone is wearing armor, they're likely anticipating being shot, unlike the armed shoppers with handguns on their person.
The guard in that store engaged and shot the shooter, the guard of any store is likely going to be the most trained person for this task, and while he put up a good effort, he was outmatched by someone that anticipated being shot at. Your answer is more people who aren't prepared to be shot, shooting at a person that accepted and prepared for the possibility that he might be shot before he even entered the store.
