WoL isn't technically a Mary Sue, he's a canon character in the fiction. I mean, you could make the argument that any fictitious and destiny-bound character falls into the Mary Sue definition, but that's just nitpicking.
WoL is supposed to be the most important character in not only the world, but in the history of the world.
WoL is supposed to be the most important character in not only the world, but in the history of the world.