Several other of the mythical story lines are depicted by Scott as having a more naturalistic explanation. The Nile turns red due to an unusually busy crocodile season. This causes the frogs to flee the bloody water and die. Their rotting corpses breed flies and pestilence. Meanwhile Moses, under his delusion, sees them all as signs of God to pursue his destiny of freeing the slaves of Egypt and returning them to their homeland.
Even the Red Sea myth is re-done. He doesn't invoke God to part it but due to a comet striking nearby this causes the waters to recede so that they can cross. I know, fortuitous timing, but he has to follow the biblical plot line. Even Moses gets caught up in the return tidal wave but by luck, or the grace of God (take your pick), he washes ashore to continue the lead on the journey home.
The linked review thinks the film is about Scott's own struggle between his atheism and how religious belief nonetheless can motive a hero to perform herculean feats for social good. If we see signs that God is telling us to free the slaves, and it gets us to do it, then so much the better for the newly freed people. Still, there are atrocities to be had at the hand of God, like killing the children on non-believers that don't put the blood of the lamb or their doorjamb. Scott doesn't provide a natural explanation for this, like his own tribe going around killing the innocent children. He keeps that one a mystery of God.