Interesting video. Good concept, but not necessarily practical as far as calling 911. Yes, you should be prepared by having your cell phone and housekeys with you. But as already stated, dialing 911 is not likely to be possible under the described circumstances.
We must be aware, but not paranoid. Part of awareness is being perceptive of body language. In the video the guy on the bench approached in an aggressive manner. Since there are also those whose plan is ill-intent but do not approach aggressively, we need to pay attention to our gut feelings. We can pick up on subtleties that make us uncomfortable without actually being able to identify exactly what it is, we just know we feel uncomfortable. It's wise to pay attention to those feelings and not dismiss them because the person approaching seems to mean no harm.
It's true that most people do go through their daily activities with a very low, to no awareness of their surroundings. Being aware, walking with confidence, head up, relaxed but with purpose, and making eye contact can go a long way toward sending the message that you are not an easy target.