Change the phrasing here to be more charitable and I would say instead:
"There is an obvious common factor that explains some of the variance: dementia would stop people from dancing; the beginning stages would likely do so as well. Thus we should expect to see a correlation between dancing and good health even in the absence of dancing being preventative. Because of this expectation, seeing a strong link may be enough to locate the hypothesis, but it is likely to be some combination of statistical outlier and reversed causality. Thus if an effect exists, it is likely to be much smaller than the first measurement."
Frequent dancing causes a 76% reduction in the risk of dementia. The benefit seems to be from freestyle social dancing, though I don't think that part is as well-verified. The benefit seems to be from making large numbers of quick decisions.
I wonder if playing jazz has similar good effects.
For something a little geekier, Shiva Nata, a system of keeping the mind fresh by doing more and more complex movements. The author of the blog is a silly person, but the system seems to be for real.
Edited to correct html which was entered in the text field instead of the html field. It wouldn't surprise me if having to deal with electronics and computers tends to prevent dementia, considering the number of fiddly and changing details one has to deal with. Or is fun required?