From the Methods section of the paper (under "study design"): "We also included meta-analyses that pooled data from primary prospective/retrospective cohort or case-control studies. These studies were the focus because of their ability to assess causality for observational research".

While "causality" is a strong word in the sentence above, the data is much stronger than simple correlation. Of course, outright causality has not been established, but the evidence to determine a predictive association is strong.