a randomized trial, which is the gold standard for establishing causal association
even then you have placebo and file drawer effects.
and adjust for confounding and other biases appropriately
from the article: "Whenever possible, adjusted relative risks were extracted; otherwise, crude relative risks and 95% confidence intervals (CIs) were calculated from the number of events."
From the paper itself:
"Forty-eight curves (908 182 subjects and 86 941 deaths) were adjusted at least for age; among them, 28 were adjusted for social status too, and 10 for social status and dietary markers."
This is adjusting for 1-4 confounders out of many possible confounders. Even doing so they lost half of the association. Note that the scientists themselves do not claim a causal effect, but only association. It's reasonable to take their word for this.
I personally would not change dietary habits just based on studies like this.
My roommate recently sent me a review article that LW might find interesting:
Personal observation says that LWers tend not to drink very much or often. Perhaps that should change, to the degree suggested by the article?
Full article here.