I interpet that in this one study it suggests that someone with a damaged heart is not going to benefit by exercise. As someone stated previously it's tough to change a near life time of bad living by doing gentle cardio type exercise after a heart is damaged by a heart attack. We are talking about a damaged heart here.
The good news is there is major studies that has shown that exercise benefits heart health conclusively. More people have avoided heart attacks than has suffered a heart attack due to exercising. Long term studies like those conducted at the Cooper clinic has reinforced that point.
Cardio decreases visceral fat levels that lead to common metabolic conditions that effects heart health. It raises HDL's and decreases LDL's. It lowers sugar levels and blood fat levels namely triglycerides. It lowers blood pressure. It increases stroke volume that is illustrated by a lower resting heart rate that show a more efficient heart. Cardio also lowers cardiac inflammation which is thought to be a major role in heart disease.
Most heart attacks in endurance athletes is due to unseen genetic predisposed heart defects. Electrical disturbances from valve problems and inherited enlarged heart walls are unknown problems in some seemingly healthy athletes. Over thirty it's usually clogs in the plumbing so to speak by inflammation and a type of blistering of cholesterol, calcium, and fat that lead to a artery clog.
Exercise is never a cure all. It puts the risk factors in your favor. The question on this board should really be is taking drugs to pose in speedos a risk to health. Most users will argue the health risks are small but I disagree.
