The speed of the interconnect really matters for most supercomputer problems. Without knowing the characteristics of that interconnect I would be hesitant to call 10,000 machines at Amazon a supercomputer.
I wonder how they'd count Folding@Home, then: 500K active clients, 6M total clients, but only a fraction of their clients are active at any given point in time.
I highly doubt it is anything better than a shared gigabit ethernet connection, which makes me doubt that it has any chance at all to get to TOP500 levels, and I doubt you could get enough of the HPC instances (where you are kind of promised a dedicated network if you get enough of them) to get 5k cores.
But you're right, if there are any interdependencies then the interconnect becomes important.