"Really? You found a use for me." He raises an eyebrow but shrugs a moment later. "But then again, that was mutually beneficial. I helped you with my powers and you gave me what I want."

Luxord doesn't know about Xemnas's lies. He doesn't like the man but he does trust his words to a certain extent. He sees no reason for him to lie to him. It's not like Luxord cares what he does as long as it doesn't involve Xaldin or Dilan.