There are a few known risk factors for heart disease; age is the big one, and then genetics and smoking. Everything else is instead a risk factor for a risk factor for heart disease or even more circumstantial. That means the authors have no real way to know, but people rarely lie about coffee. They may under-report consumption but people don't outright lie about drinking coffee the way they lie about smoking or drinking alcohol or eating late at night. Coffee is a socially acceptable form of ingesting a known toxic carcinogen.The good news, if you want to believe food frequency questionnaire results, is that coffee was associated with lower risk of heart disease.