Am I missing something? These people had a weeks notice of this and they didn't bother to fill the tank before leaving?
Yes, you are missing something. Even people leaving with a full tank were runnning out of gas after spending 20+ hours in traffic.
It took my son 24 hours to drive a 3 hour drive from Houston. He has a fuel-efficient car, and a full tank before he left, but still almost ran out of gas just idling all that time. In a gas-guzzler, he would have run out on the road from Houston to San Antonio.
Being stuck in traffic for a dozen hours will put you on empty