The Primarchs doubted the Emperor many times.
Those doubts led to the Heresy, then led to the Imperium suppressing doubts through faith and violence.
Leandros doesn't question his place or the system, he champions it. He dare not doubt, for the system demands that he cannot.
"He (Roboute) is a demigod out of time. Fighting for a golden age that will never dawn again. The Primarch Reborn demands the impossible."
Why is Titus dressing down his own Primarch?
So, Leandros is right, after all?