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.
Its unlikely, but not impossible and when you're dealing with preproduction prototypes you are holding someone elses valuable proprietary information, which in the case of this waterblock is unlikely to be something thats worth copying as its a high cost low margin niche product. But someone could.
Its not like it was just expensive or just someone elses property. It was someone elses one of a kind proprietary design prototype, they knew this and they STILL couldnt show the proper care and responsibility. Thats why this admittedly unlikely edge case is a big deal.