So, yes, making a GPL request will work for the very few components still under GPL, if a vendor releases a binary patch. But for most things outside of the kernel, patch diffing comes back into play, just like on every closed-source OS.
I would understand in a modular system like an operating system: one can argue that the kernel is a single component.
But if you're buying an appliance, the OS is effectively one single unit: all linked together.
Why does a binary executable and a binary image seem to operate differently in this space - both are inscrutable?
Either a) when the license has an explicit exemption (such as at glibc or the kernel's userspace interfaces) or b) when something ceases to be a "derivative work" in copyright terms (which is ultimately a legal question for lawyers).