This is canon.
Lucius surfing a Screamer of Tzeentch is canon.
Quote:
Lucius and Sanakht had already departed the Khemet, leaping from the embarkation deck to ride the singing manta-creatures to the surface. They made for Sanakht’s blaze-topped tower, to hone their swordplay in preparation for the day when they would try to kill one another.
Graham McNeill, The Crimson King