This is a good analogy. If you look at a modern mainframe CPU it becomes pretty clear where the differences lie. Fewer, beefier cores with a lot of focus on cache.
Supercomputers tend to use more conventional cores, but way more of them, and connect them in a large fabric. There's a lot more focus on parallelization and horizontal scaling.
Mainframe overall compute is nowhere near a supercomputer, and you probably shouldn't be running a massive physics simulation on a mainframe, but you may get more consistency and reliability for well defined tasks.
