Apparently they are using on the density of thorium to generate heat.
"Density to generate heat" is a meaningless term.
Thorium has a half life of 14.05 billion years. Real-life RTGs mostly use plutonium with half life of 87.7 years and thermal efficiency of 0.54 kW/kg (assuming 100% efficiency you'd need ~140kg of plutonium to get equivalent power output of a low-end car engine). Basically thorium simply doesn't have enough passive heat generation to be used as RTG. If they have an additional neutron generator to shoot at thorium it could be used but I have hard time seeing such a thing in a car-sized box.