It’s an interesting concept. I wouldn’t say that it’s useless out of hand, but … it’s only useful to the extent that people cannot or will not take responsibility for their own defense. She could just as easily have pulled a gun from her purse as her phone. Why should she not be allowed to do that instead?
Even if the app is real, and she can get the phone out of her purse in time, turn on the phone and go through whatever screen lock she has, activate the app, get a response (because as the video said, that’s not instantaneous even when calling 911), get an adequately lighted video / photo of the perpetrator, have the respondent say something in a language that the potential attacker understands – and can hear over ambient noise – and assuming the perpetrator is ambling along as this one did (most perps seem to prefer a fast strike after picking their target, not this kind of walkabout scenario), and the perp is not drugged, desperate or drunk too much to understand and make a rational response … hello? Rational response? If he was rational he would not be stalking her in the first place. Even if all of the above occurs … the guy may decide, “the hell with it, I want that purse / that car / that ass” and then it’s just another unsolved crime, but this time with a video. Meanwhile, she’s out a purse, a car, beat up, raped or dead – or all of the above.
She could have pulled a revolver, pointed it with two hands and said, “Decide.” Depending on his decision (and her own level of training and resolve) the issue is over in two seconds or less, with her walking to her car with “another success story for armed self-defense” under her belt.