As someone who works in cardiopulmonary rehab, weight training can improve heart function, but the adaptations are different from aerobic work. The lowering in risk isn't so much from improved heart function, but improved skeletal muscle function (strength) reducing the workload on the heart during physical activity.
Cool discussion. Could you say a little bit more about how improved muscle function reduces the cardiac load? I remember Maxwell saying something about that regarding lower-body movements promoting circulation.
Let's put it this way, who's heart/CV system is going to work harder with a given weight? The person squatting 100 lbs for 12 reps who's max is 200 lbs or the one who's max is 300lbs?
Lower body training (not just strength training) promotes venous blood return to the heart (especially the calves).
As far as the heart, strength training thickens the heart muscle walls and allows it to squeeze harder. Good in some situations, not good in others. Over time this can lead to stiffening of the heart wall and not allow for the ventricle to relax and fill with blood properly.
This is where proper aerobic training comes in. The size of the chamber (left ventricle) will increase and allow the heart to fill with more blood due to relaxing more when filling. This is known as increased stroke volume and typically results in lower resting heart rate.
The heart must maintain cardiac output which equals stroke volume x heart rate. Increase stroke volume and heart rate will drop to maintain the same cardiac output.