The basis of derivative work, even when RMS was bullying projects with it, was whether your code was dependant on the GPL implementation, or say, could just as well be built with another vendor's library - even if that one was GPLed too, so long as it wasn't derivative of the first runtime.

Essentially a case of "coding for standard interface offered from multiple sources where the interface itself isn't GPL" vs "Your code requires rework in order to work without this specific GPLed dependency".

This is all grossly simplified, but among other things, it means that separating GPL code to another binary you talk over any form of IPC doesn't necessarily mean you're avoiding GPL.