That doesn't necessarily account for people getting car sick on long, smooth, winding canyon roads driven at constant speed. The jerk induced by following the curves in the road should be lower than that caused by braking.
Honestly I think it is a combination of where your awareness is focused, and jerk. Long windy roads put varying lateral forces on passengers which probably become an issue if their focus is in the car.
Sometimes I get "carsick" while working with very small components (such as eyeglass-sized screws) that aren't behaving as they should, so I would have to attribute greater weight to the idea that carsickness is caused by a disagreement between the senses.
