In Angels of Darkness the message its trying to put across wasn't that Luther was loyal, its that Lion El'Jonson was disloyal.
What Angels of Darkness claims is that Lion El'Jonson was corrupted by Chaos and deliberately arrived late to Terra so he could see who was winning. If Horus had been winning then the Dark Angels would have joined with him, however they lost and when he returned to Caliban the loyal garrison and Luther wouldn't allow them to land, believing they were tainted. Jonson opened fire on Caliban and the battle begun.
I personally believe that its true. The Fallen are really the true loyalists and the ancient Dark Angels were the traitors to the Imperium, even if they didn't actually defect.
What Angels of Darkness claims is that Lion El'Jonson was corrupted by Chaos and deliberately arrived late to Terra so he could see who was winning. If Horus had been winning then the Dark Angels would have joined with him, however they lost and when he returned to Caliban the loyal garrison and Luther wouldn't allow them to land, believing they were tainted. Jonson opened fire on Caliban and the battle begun.
I personally believe that its true. The Fallen are really the true loyalists and the ancient Dark Angels were the traitors to the Imperium, even if they didn't actually defect.