This reader gets it:

Much of the discussion of “historical accuracy” misses the point. As Mel Gibson has pointed out in numerous interviews, he believes (rightly in my opinion) that no Hollywood treatment of the crucifixion has have come close to accurately portraying how brutal it was.
His goal is to simply to show how awfully Jesus suffered for the sins of the world. To appreciate the horror of Jesus’ death, conservative Christians look not only to the Gospels but also the Old Testament messianic prophecies in the Psalms and Isaiah.
When Gibson says he is striving for historical accuracy, this, above all is what he means.