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.

Given that there was no concerted effort to move either of them, my answer is "of course not! They are correlated"

humans have become less active → our intelligence has suffered as well

Eg: Less active = more weight gain = higher likelyhood of GERD and sleep apnea = less sleep = lower attention span and memory.

Boom, lower IQ because of lower fitness.

More accurately, intelligence is negatively correlated with fertility in women in the Western world. The correlation is still positive for men.

Generally it's believed that the negative effect on fertility is due to intelligent women getting more education, rather being a direct negative effect of intelligence.