Meanwhile the back is to some degree supported by the seat back so pressure would tend to remain more or less straight down the backbone, particularly with a sudden stop from a mostly downwards and slightly forwards motion.
I think the body would have flexed forward with impact - assuming it wasn't already as per brace position. There was some forwards movement which stopped with impact. I know we have been talking about lowish forward vectors, but it would still represent a reasonably high speed car crash. Plus the top half of the body had to go somewhere when the bottom half stopped. It had a lot of momentum, the body can't simply telescope downwards into itself, and the muscles would not be sufficiently powerful to stop this forward flexion.