Typically a good design has an energy absorbing crumple zone around a rigid passenger compartment.
"... filled various body cavities in the Pinto with rigid urethane foam."
Without seeing the article it's not clear what they did, but urethane foam would also be an energy absorber when it is deformed. It is possible to create polyurethanes with vastly differing flexibility. When they are foamed, their properties are vastly different. Rigid polyurethane foam is the type that is used for insulation (in refrigerators for example) and flexible polyurethane foam, 'foam rubber' is the type that is used to cushion seating.