This reflexive cynicism is anti-science, because what you claim is not what's happening here. The authors have a sensible causal hypothesis, which is supported by an empirical correlation. A lot of the value seems to be the hard work they did collecting years of urban rat data from various different countries.

Individually, sensible causal hypotheses and correlations aren't worth very much. But when they align and the hypotheses are supported by additional evidence, that's usually a good indication of real causality.