I think it is sound advice. I mean a pound of lead weighs the same as a pound of feathers. It's still a pound. When people say muscle is heavier, what they mean by that is someone can gain 2 pounds but their clothes fit looser, and it doesn't make any sense to them. Muscle is more dense, so a pound of muscle takes up LESS space than a pound of fat does, so you can get smaller AND gain weight.
I do think a lot of people use it as an excuse when the scale doesn't go down, but if the inches are coming off and the scale isn't moving it makes sense that someone is both losing fat and gaining muscle.
I don't like the fact that it says most women don't do enough resistance training. While it is true that many just do cardio. I know for me, I can gain muscle very easily. I always have been able to, and it is not uncommon for me to go down a dress size without losing weight if all I'm doing is resistance training.