If a brain does focused relaxation, it gets better at relaxing. I don't see how this would have been very counterintuitive 50 years ago. The unexpected part was the causation going from mental actions to brain anatomy.

Why is relaxation something to get better at? Why isn't it just the absence of effort? And even if we assume that it's a skill, why meditation and not, say, watching The Ed Sullivan Show? Plenty of people found that relaxing.