I disagree. Even though walking burns fat at a higher percentage than glycogen, it's still burning a lot less calories than running or even doing HIIT. Even if glycogen is mostly burned during exercise above 70% intensity, it's still burning much more calories than walking, and even with the fat not being burned much the higher calories burned still makes the fat higher. Furthermore, the body conserves the energy source that it used during exericse and burns it's counter-part after exercise. Meaning, if fat was burning primarily during exercise then glycogen will be burned afterwards, and vice versa. That's why HIIT is so good because since it burns lots of glycogen during the exericse and since it raises your metabolic rate, insulin sensitivity, and FFA oxidation, the body burns mostly fat for up to 24 hours after exericse (if you do it hard enough).
Walking is definately fine to add to a cardio routine that's low impact but it shouldn't be the main cardio exercise. However, I can't speak for those people that have 230+ pounds of muscle - when low intensity cardio is preffered. But even at that point, walking really is only helping a little due to the fact that with all that muscle theyre burning fat like crazy all day because of all the muscle around. For those that aren't like them they shouldn't train like them.