I can confirm this. The first several times that I played FO3, I did this quest fairly early where I either didn't or couldn't sneak very well, Arkansas spotted me, cars got "sniped" when I approached them and blew up while he tried taking my head off directly as well.
Eventually one of the last times I did it at a much higher level at night when my sneak was maxed out. I was pretty much a ghost. Took my normal way in... and much to my confusion, cars start getting "sniped" and exploded. I make my way up to him without being shot at once, and find he's asleep. At that point I realized that the car sniping was indeed a scripted event. Personally I wasn't too bothered by it being scripted though since it added a clever little thing to the quest.