As far as I know it was never a sham, actually. I guess Riku said that out of despite/jealousy, or because he knew there was something strange about Xion the moment he looked inside her hood and suspected about her true nature. He was aware of replicas being a thing, after all.
I don't remember which ultimania or interview but Nomura said both Roxas and Sora wielded Sora's keyblade at the same time. It can be applied to Xion as well, if the keyblade can exist in several places at the same time it means it's not fake. Xion only copied the ability to wield it, not the weapon itself. It's not like the org. XIII created a fake keyblade for her. If her keyblade was fake and not the same Roxas wielded she wouldn't have lost it at one point in the game. It is implied that happened because Roxas and Xion shared Sora's power, Roxas was stronger at that time and that weakened Xion to the point of being unable to wield. After going to missions together they balanced their power and Xion recovered the keyblade, and later in the game Xion was stronger, weakening Roxas. Sora's power was constantly fluctuating between them.
I think we can assume that when she returned, recovered her heart and became her own person she got a keyblade of her own, just like Roxas got two.