Okay, let me take a shot at this, because I believe I understand it.
The legend does not mean a legendary Pokémon; it means an actual legend, about the legendaries, which he put in the first post. And that legend - the story about the legendaries - is one which is only told in this region. Probably passed down for generations.
And when he says light, I'm pretty sure he means it not in the physical sense of light and darkness, but instead meaning the same concept as good and evil, or right and wrong. He means, I think, that these are the Pokémon that fight for justice and for the good of humans and Pokémon alike.
:D But I could be wrong.