I was never quite sure if that was a deliberate ploy. The demonstration showed that it quite possibly had enough strength to stop a bullet (it would just smash).

The failure of the demonstration got vastly more coverage than if it would have if the, um rock?, just hit the window, went donk and fell down. If your goal was to communicate to the maximum number of people that you had put bulletproof glass in your cars, it would be quite a reasonable tactic.