Eh, I wouldn't call it nonsense. Endomorph, mesomorph, and ectomorph are just words used to describe the three basic body types. It's no different that saying there are three hair colors: blonde, brunette, and redhead. Obviously, there is a lot of variation and not everybody is going to perfectly into one category, but it's an okay way to generalize things.
As for building muscle, it's not bad advice. The more muscle you have, the more calories your body will naturally burn, the less bodyfat you will have (assuming you're also eating a healthy diet). It's Fitness 101.