I AM the Evil Twin!
He gets Lore of Light because he was a Liche High Priest before he became the Supreme Lord of the Undead, in fact he was one of the most senior Liche High Priests in the days of ancient Nehekara. The magic of ancient Nehekhara used by the Liche Priests was based around incantations that summoned the power of their deities which were based around the sun and the afterlife. AS such they could use light magic by channeling the power of their deities.Edit: Also, why the heck do undead guys get Lore of Light? Can someone please explain that one to me? Can't get shadows, but we got that goody good lore!
In game terms this is represented by the fact that Liche Priests have access to the Lore of Nehekhara, Lore of Death and Lore of Light.