Um, not quite. Although the autopsy findings were never released in full, basil skull fracture is the most commonly accepted cause of his death (not uncommon in racing). Seatbelt failure was speculated, but later thrown out.
The blunt force trauma to the soft internal tissues is what generally kills when you have such severe deceleration, followed by breakage of the cervical spinal area with brain stem trauma.
This footage also seems to have shown the vehicles striking the barrier at a slight angle, allowing some of the kinetic energy to be transferred into sideways motion. Although if you were in these vehicles, you'd probably be pushing up daisies anyway.