umm, many supercomputers were built by taking existing components and hooking them up in much the same way that this thing was. special hardware is optional, not an inherent requirement.
if a system is only a supercomputer if it can handle the most demanding challenges of the day, is something 1/10 the power of the most powerful device still a supercomputer?, what about 1/100 the power?
note that the "top 500 supercomputer" list has the top entry with a rating of 16324 and the bottom entry with a rating of 60. how can a machine so weak still be considered a "supercomputer" according to your criteria
the Raspberry Pi thing being discussed here is a cluster of machines, not a single machine. It has separate memory, storage, network, etc for each node. This is a vastly different environment to work with than a desktop system.
No, this cluster isn't going to do any groundbreaking research or solve any "large challenge" problems. But as something to teach people about supercomputers and let the experiment with and learn what does and doesn't work for HPC computing, this is a much better thing to use than an equivalently priced SMP system.