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.
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.
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.