If you think about this logically, you are somewhat right.
The first part of the sentence is certainly true for a larger proportion of time than it is certainly false. How many random terrorist attacks has a bystander with a gun managed to stop? How many random terrorist attacks can you say for certain will be stopped simply because there was a guy/officer with a gun nearby?
Shooting a machine gun in a packed square during Christmas is pretty stupid thing to do once a terrorist has probably already blown himself up at the other corner of the square. I mean it just doesn't make sense. In this situation it simply doesn't make sense because an attacker could walk right up to anyone and push a button and they wouldn't have a clue let alone any time to turn off the safety. They don't have some sort of magic senses. This is why it's a stupid to assume they're there to stop a terrorist attack; this is real life, not a movie. To "stop" a terrorist attack you need to have prior intelligence (this does not justify nor require having random officers with guns), and then again it would be an utterly disastrous idea/charade to decide to start "stopping" only once they've gotten to the middle of a packed square.
Not sure about the uprising part though I think we should save that for another thread.