It's not a surprise to anyone in fuckcars, but to wider audiences it could just be a matter of them never considering it. Our parents had cars, now we have cars, cars are used to go places, even if I get stuck in traffic it's worthwhile to get to my destination - end of thought.
Sure there's a huge group of 'carbrains' but in my experience, most people don't have an allegiance one way or the other.
Weirdly, the abstract on the original paper is a bit clearer than the linked text:
Among children who took inactive modes at baseline, 42% of children in London and 20% of children in Luton switched to active modes. For children taking active modes at baseline, 5% of children in London and 21% of children in Luton switched to inactive modes.