11 - 15 years is exactly the period of the "standard business cycle", which was established by the boom/bust nature of economies that depended primarily on agriculture. Farmers considered it common knowledge that the "business cycle" depended on the Sun's variability.

So, there is a definite, well-known, historically traced effect that occurs on around that time scale. It would take serious study to move past the coincidence level, but it's not something that can be dismissed out of hand, either.