People who eat fish more than 2 times per week live in coastal areas where sun overexposure is most prevalent.
I was thinking along similar lines. People who love fish might tend to fish more, being outside.
Or is the study controlled for that?
I’ve heard that generous amounts of sun exposure and its effect on Vitamin D levels helps lower the risk of all cancers except skin cancer. But I know all such assertations are controversial and will be challenged by other studies.