Quote Originally Posted by Ulysses S. Grant View Post
Emphasis on this part of my post you quoted:

"what is as close to fear as a marine can feel."

They recognise the feeling but it doesn't affect them. This feeling is stronger when you throw a Primarch in the mix.
As far as i recall back in the day only Black Legion was the only SM heritage legion with unbreakable morale. After that all SM legions became unbreakable (unless that changed haha).
Writers mix up the rules and even make up their own. Never read too much into fluff.