It is a fair point that there are different degrees of exercise though. I have been debating whether to count my commutes as exercise too, as it is much less intense than other things which I would count as moderate exercise.
For example, I do my longer runs at a pace where I can still hold a conversation. My heart rate is around 140-150, from a resting of 65-70. I'd say that was moderate exercise (unlike, for example, running sprint intervals, which would be intense exercise, with a heart rate of 160+).
However, when I cycle to work, my heart rate gets nowhere near as high as on my easiest runs, except on 10%+ hills. It's usually about 110.
Now, I am not saying that it is not worthwhile - there is a lot of benefit in training in all the different heart rate zones, including the lower ones. But I would say that is low intensity exercise really.