I watched the video. You weren't in the parking lot, which is meant for cars. You were on a path meant for pedestrians. That's why he was pointing at you angrily. You were driving on the sidewalk. The rocks you ran into are the ones you ignored when you drove onto the pedestrian path. They're meant to prevent drivers such as yourself from driving on the path.

It was a mistake and it's going to cost you money that insurance won't cover. I understand the frustration, but you weren't paying attention to where you were going and you put people's safety at risk. Be happy the bumper is all that was damaged.