Because Guilliman has missed 10k years, is trying to rewrite history and is still suffering the trauma and grief of the heresy, being killed by his own brother and living with chronic pain and the suspicion of people thinking he is trying to usurp the Emperor.
And Titus is smart
"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?