A supercomputer is any mechanism whose performance capability, either by design or by default, enables it to compete — effectively or otherwise — in the market for functionality and information. There have been times throughout history where handfuls of spare processors, cobbled together with homemade substrates, produced supercomputers. And because they were super, they qualified.
Today, supercomputing performance cannot be achieved accidentally. It must be designed willfully, intentionally, and with a modicum of tolerance for both corporate and international politics.