I am a so-called "fitness buff" and I've never said anything about disc golf not being decent exercise. In fact, one of my soldiers who was on the verge of being booted from the military because he was too heavy was able to stay in because I introduced him to disc golf. He had let himself get to the point that more strenuous exercise was nearly impossible as his joints would hurt, his muscles would hurt and he would easily vomit. Taking him to some of Colorado's more challenging courses helped him quite a bit.
Any physical activity is good. Disc Golf is fun, cheap and easy to play (yet difficult to master...hooking lots of people) and is much more beneficial than just sitting around all day. Does it help me as much as my gym sessions or a nice, long run? Nope. But it's way healthier than what a vast majority of Americans do...which is nothing.