I don't see any evidence that this is causation and not just correlation. I'd believe that people who sit more are at higher risk for heart attack. But I'd also argue that those people are more likely to get less exercise.
But now, researchers are beginning to suspect that even if you engage in regular exercise daily, it may not be enough to counteract the effects of too much sitting during the rest of the day.
That is not what the researchers suspect. The article misquotes the study. You only have to go as far as the abstract to find: "Regardless of the amount of sedentary activity reported by these men, being older, having normal weight, being normotensive, and being physically active were associated with a reduced risk of CVD death."
But now, researchers are beginning to suspect that even if you engage in regular exercise daily, it may not be enough to counteract the effects of too much sitting during the rest of the day.