I know that seems like a silly question since we all have so much experience with the concept, but if you look at some of the research methods for the “cardio” exercise you'll find that the studies were clearly pushing the subjects into an anaerobic energy production. Without getting too technical, this means that the gains realized in the study were from a higher intensity. An intensity that was beyond aerobic or “cardio”.
Aerobic exercise is not defined by the activity, but the intensity of that activity. Pedaling a bike at a resistance (indoor bike) and speed that you can sustain for at least 8 minutes can be considered aerobic. Pedaling that same bike at a resistance and speed that you can only sustain for 1 minute, is not aerobic, it is anaerobic, even if you repeat that minute multiple times with rest in between. But what if, instead of resting between bouts of the high intensity, you continued to pedal but at the lower resistance and speed? This is, of course, the idea of interval training and it has many benefits. It is a stretch, though, to call it cardio exercise.
This matters because of how the public uses the information from the fitness industry. In 1996 the Surgeon General’s report on physical activity gave recommendations for the minimum amount of exercise necessary to decrease risk of disease. One of these recommendations was that exercise could be broken into shorter bouts, as low as 8 minutes long, and that quickly became counting all movement toward exercise goal regardless of how brief. Another recommendation was that exercise should be most days of the week, which somehow translated into 3 days a week. And remember, the recommendations were the minimum to have any benefit. So, when they were lessened even more it is no wonder that the benefits also failed to be realized.
So, what we know is that exercise can make a huge difference in many areas of life and there is a clear dose-response relationship with greater benefits to be gained with higher intensity and greater frequency. Using this as your guide will be far more effective than some classification label that can lead you to believe that all of those listed benefits of cardio can be gained by walking.
As always, let me know how I can help.