This is simply not true. Solar cars are made of composite materials and have composite or metal frames. These materials allow them to be extremely light but they are still very strong. I have personally seen the results of a lower speed rear end collision between an SUV and a solar car (solar car was stopped, SUV rear ended it). There was almost no damage - the SUV was stopped by the composite body of the car - it never even impacted the frame ebcause the body was so strong. The yale solar car has a carbon fiber lower shell with a kevlar top and a titanium frame surrounding the driver including a titanium roll cage. Other cars are built similarly.
Hitting an object with a combined speed of 120mph will destroy any car. The fact that the minivan wa only minimally damaged is just a testament to the fact that the solar car was light and didnt have the energy to damage the minivan. This is a GOOD thing - If it was a head on collision between two minivans, both drivers would probably have been killed.
If you can't see why heaviness is a bad quality for a vehicle tha is going to crash, just look at any train wreck. The trains are usually going less than 60mph and they do a LOT more damage when they collide because they are HEAVY.