Gary lied to us. Gary kept the secret well hidden.
Tradition is a lie. Behold the magician, and notice how his hands point both Up and Down. White and Black magic. Right Hand and Left Hand Path have to coexist in the Magic User. Restricting oneself is imbecile. Knowledge must be ultimate. Nothing must be hidden. Nothing must be Forbidden.
MUs unlock the secrets of creation, walk on the Sword across the Abyss, destroy their former selves and discover the names of G-D and know the path to the Godhead.
Magic Users can cast all spells. And can wear armours. That “can’t wear armour” in every D&D handbooks is just part of the biggest cover-up in the gaming industry.
The Left Hand Path spells both require a superhuman hubrys and cause an otherwise impossible sensitivity to metal, neither of which is conductive to clad oneself in iron.
The Right Hand Path only requires intimate knowledge and correct pronunciation of the Names of G-D. Iron has no power to stop invokation. Iron bears the Magician across the Abyss. Iron does not stop G-D from hearing. While Clerics don’t understand the nature of G-D, but can just beg and grovel for the tiniest speckle of their power, Magicians demand and are satisfied.
The Left Hand Path is the weakness of their flesh.
The Right Hand Path is the power of G-D.