I didnt see anywhere that it was the "only" prototype. I believe it was their most complete or their pre-production prototype.

So likely a hand machined and finished item as opposed to something made with production tooling or in batches to reduce manufacturing cost. Something that could also be sent to reviewers as its essentially what the finished product will be. So not only are they out their best prototype, but the opportunity cost and the potential that someone like EK could have bought it and reverse engineered it to release a competing product with little to no R&D cost.