It's not the item that's the key though. It's the true friendship and emotions that it represents. I.e. Rarity's thread is not the key, it's her friendship with Coco Pommel (who incidentally GAVE her the thread). If Coco had given her something else, say a ring or something, then that ring would be the key. The same thing goes for Pinkie with Boneless. Boneless himself wasn't the key, it was Pinkie's friendship with Cheese Sandwich. If he had given her something else, then that would have been the key.
So Tirek didn't ACTUALLY have the key. The key was Twilight's friendship with Discord. The medallion was only the key because it is what Discord gave her to represent their friendship.