The Bitcoin network is indeed more powerful than supercomputers, but only if you compare the Bitcoin mining performance. Since this is the only thing that matters in the context of Bitcoin security I think the comparison is valid.
Exactly. It doesn't matter how good the whole earth simulator is at modeling weather patterns or how badly the Bitcoin network would compare on that task. What matters is how good would supercomputer be at mining and thus how likely are they to represent a credible threat to the Bitcoin network.
The reality is supercomputers aren't so "super" when it comes to mining power. They are totally outmatched by the aggregate of legit nodes in raw integer performance. Even among supercomputers which have decent integer potential they are outmatched by the sheer number of Bitcoin nodes.