I suspect this is correct, and has to do with the intensity of the demand on the heart. Many things are thrown into the "cardio" category which use the heart, of course, but make no constructive demands on it. Walking, hiking, speed walking, swimming, and jogging are like that.
HIIT exercises, on the other hand, do stress the heart because of the intensity of all kinds of sprints. 30-60 second maximum sprints of any form are good stressors. So is moving heavy weights. Heavy, not light weights. Sets of deadlifts reaching up to 70% of your max is a serious stressor on the heart. When you finish each set, you feel it in your heart. And you feel a bit dizzy from that stress.
What true cardiac training does is to stress the heart to the point that it is forced to build its muscle and to grow new arteries. Those new arteries can save your life when others get blocked up with gunk. If your breath can keep up with your exertion, it's not real cardiac training: it is endurance training.
What most people term "cardio" is endurance maintenance and endurance training. These are important in life, but have little or no heart consequences for the otherwise healthy.
Here's the article (via Instapundit): Weight lifting better for heart health than running, new study finds
Our Maggie's Fitness for Life program (heavy powerlifts, endurance cardio, HIIT cardio, calisthenics) incorporates all fitness aspects: Muscle and Bone strength, Endurance, True cardio, and Athleticism. We are convinced that's a balanced program for vigor for all ages.