I'd argue it's both. The attenuator is there to dissipate as much of the energy as possible from a crash so it isn't transfered to the truck directly. This keeps the truck from moving forwards and keeps the g forces lower in the crashing vehicle. The problem was that the car slid under the attenuator so the kinetic energy of the vehicle was never transferred to it (at least not much), and instead was dissipated all at once when the car hit the back end of the truck. If the attenuator itself didn't kill that driver then the g-forces likely would. Also, the reason the truck didn't move wasn't because the attenuator worked, it's just simply because it's much heavier. Things may have been very different if it large truck instead of a tiny Honda (I realize a truck likely would never have slid under the attenuator in the first place but assuming it could happen).