I think what the drivers ed textbook is asserting that the acceleration experienced in both collisions is equivalent. I think the assumption that it is not mentioned is that in the running into a brick wall is an inelastic collision, while the two car's hitting each other is an elastic collision. So in one case the acceleration is from 100 -> 0 km/h while the other is from +50 -> -50 km/h.

I have no idea if that is true in reality, but to me that seems like the most likely explanation. I agree, without this the statement does not make sense.

As a side note on the article, I'm not sure why David Friedman associates being open to being questioned by their children with libertarian economics (with circumstantial evidence at best)? I, myself and many of my friends have been educated in a similar way and our parents were definitely not libertarian economists. In fact I would argue that this philosophy is routed in anti-authoritarian education, which originates in communities more associated with anarchistic/socialist political/economical leanings.