If the smokers who drink caffeinated beverages are less likely to develop heart disease than the ones who don’t drink such beverages, this is evidence there’s actually a negative correlation between drinking caffeinated beverages and heart disease among the people who smoke.

The argument doesn’t concern what causes heart disease. The author is simply trying to establish a correlation on the basis of related correlations. But the author doesn’t assume that anything is the cause of something else.

This possibility doesn’t help show why caffeinated beverages might not be positively correlated with heart disease. If anything, it does the opposite, suggesting that there might be a positive correlation between caffeinated beverages and heart disease.

(D) might show that genetic factors play a causal role in heart disease and undermine a hypothesis that caffeine consumption causes heart disease. But (D) presents evidence of a positive correlation between caffeine and heart disease, which supports the author’s conclusion.

The existence of a common cause doesn’t undermine the possibility of a positive correlation between heart disease and caffeinated beverages. If anything, it supports the existence of a correlation by explaining why they might be correlated.