The steps he had to take to turn it on and actually exit the vehicle without it turning off automatically was pretty comical though (TL;DW for anyone not watching):
1. Clip his seatbelt behind him, so he could keep it attached after getting out of the seat (undoing your seatbelt turns off autopilot)
2. Start driving at a low speed, turn on autopilot, then bring it to a stop
3. Climb out the window (as the doors didn't seem like they would open while the car was in motion)
4. Use a stick/tripod to tap on the acceleration pedal to resume autopilot driving
And even then, the vehicle stopped like 10 seconds later when autopilot got confused about its surroundings.
Even if the seat needs weighted down, this seems like a pretty trivial thing to do (put a backpack or something in the seat) if you're already going out of your way to intentionally trick the car in so many other ways.
Since the car didn't even have autopilot as a feature, it seems way more likely the riders just went with the age-old "prop the pedal down" method that's caused accidents like this since the beginning of cars.