DROPPED items are not the same as STOLEN items.
Think about Brachiosaurs for instance. You can steal Ribbons from them, and as long as you use Control on them with Relm, it's a sure steal because you can just take as many turns as you want to do it. But they DROP very, very rare Three Stars, also known as Economizers, and there is no way to steal them from them - you just have to keep on killing and killing until one drops. So if the guide says the item is dropped, there is no point trying to steal it because it's not the same thing.