And after Luxord went away asking Xigbar who he really is, Xigbar said: "The question is, who are you Luxord?" But I dunno, him being the clueless player character who just got sucked into more shenanigans fits better with Luxord, imo, than him being this sinister, seemingly all-knowing Darkness. Xemnas said, the members 9-12 where chosen to awaken the ancient Keyblade Legacy that slumbers within them. That means Luxord must have lived during the events of Union X but lost his memory just like the others. Maybe he knows who Luxu is but he doesn't know WHO Luxu is, as in he doesn't know what Luxu's current body is.