states that a supercomputer
is "a device for transforming a compute bound problem intro an I/O bound problem
Of course, some problems are compute bound, with little need for communication to take place between various parts of the computation. Such problems don't require a traditional supercomputer; Beowulf clusters have been used for them in recent years. In breaking RSA 129, for instance, many computers were used to perform an initial step; the final step, however, was inverting a matrix of huge size, and was performed on a supercomputer.
Real supercomputers have massive I/O capabilities, to help with the problem in the quip.