That is an entirely incorrect statement by all accounts of his cause of death. There have been CART and IRL crashes that have logged over 100g's at impact, and the driver has either walked away or survived after recovery. Rally cars (which are more like NASCAR's in their construction) have incredibly hard impacts with odd shaped objects and routinely walk away. Sometimes they don't, but that's because intrution into the safety cage, not injuries due to deceleration.
The cause of Earnhardt's death on multiple investigations has been listed as basilar skull fracture caused by his head not being restrained. Your body stops moving and your head is still going 180mph, so it snaps with such a force that the base of your skull fractures from your neck. A HANS device is worn to prevent this type of injury.
There was contention that the seat belt tearing allowed his head to contact the steering wheel, causing his death. But an investigation found that even if his head didn't contact the steering wheel he would have died. Also know that his lap belt tore because of improper installation. Dale thought the belts are uncomfortable so he had the crew modify their mounting.
There's plenty of equipment that would have saved his life.