Correlation does not allow you to draw conclusions about causation.

And there are other plausible explanations. Intelligence is highly heritable and negatively correlated with fertility. Also, there has been an overall increase in measured intelligence since IQ tests were first invented. Nobody really knows why. But this could be a reversal of that trend. And, importantly, people have been getting more sedentary during the entire extent of the increase. So it would be hard to square that with the hypothesis that this recent decrease being caused by less activity.