I'm not entirely sure about this, but I don't think the firefighter did anything wrong- it seems like the car was in neutral, and the force of the hose started it rolling, at which point he tried to stop it, but couldn't, because... well, you know.
I believe proper protocol would include chocking the wheels, as brake failure is non unheard of in a burning vehicle. But given the number of videos and news stories about these exact accidents happening would indicate it is an often overlooked protocol...