This is a false analogy. It's quite straightforward.
Running bash (via exec()/fork()/spawn()/etc) isn't the same as (statically or dynamically) linking with its codebase. If your MIT-licensed one-liner links to code that's GPL licensed, then it gets infected by the GPL license.
I've seen people use IPC to workaround the GPL, but I've also seen the FSF interpretations claiming that is still a derived work.
I don't know if this has ever been tested in court.
My interpretation of their FAQ[1] on it is that shelling out and IPC are fine, while linking is not. As you say, it's ultimately up to the courts to decide on.
[1]: https://www.gnu.org/licenses/gpl-faq.html#MereAggregation
you are correct. its about linking as in LD does it, not conceptual linking.