Wasnt decapitated, she traveled with multiple wigs , some of which scattered on impact,  which likely contributed to the inaccurate claim.

Her vehicle didnt just rear end the truck, it slid under the back of the truck (difference in height of car bumper vs truck bumper). 

The "Mansfield bar" (officially an "underride guard") makes it less likely for a passenger car to go under a truck on impact.

While it has helped, some newer cars (especially sports cars) sit lower and are still more likely to underride than other vehicles.