The authors discuss this in the paper's Discussion.

The prevalence of ramen restaurants is suggested as an indicator of general food preferences in the regions studied. Regions that have a higher prevalence of ramen restaurants also tend to have a higher salt intake in general. This can be seen by the increased use of soy sauce and miso compared to other regions in Japan (from personal experience, these regions are famous throughout Japan for their high-salt diets).

So I'd say that the authors are very careful to clarify that they are not implying any sort of causal relationship between ramen restaurants and stroke mortality, and I don't think anyone reading the paper would assume that either.