An iPad does resemble the supercomputers of yore, between its superscalar, vector processing (GPU), and multicore architecture.
The Parallella brings distributed-memory programming in, which is a very important development.
You and I strongly disagree on the meaningfulness or definition of the term supercomputer. Here's an easy definition: A supercomputer is any single, unified, computer system that is currently one of the fastest 500 in the world.