Well they did make a real Buster sword that weighed only 7 pounds. And that's with real metals. Granted it was mostly aluminum. But I would imagine they have other metals available in a fantasy world like that. So really it's no big deal to me. If it were made out of steel or iron, for sure. But as it stands I've no problem believing it. Especially if they attach some materia to it. I'd imagine along with whatever magical properties it'd embue, it would probably make weapons insanely easy to weild no matter what they are.