Alright Richard Stallman, why'd you have to kill Aniday and wear his online persona as a suit?
More seriously, I don't think it matters:
GNU isn't an OS*, it's a collection of programs (including shells to allow interaction with the kernel) that sit on top of the Linux kernel. Likewise, as you say, Linux isn't a full OS either. Each of them are modular halves of an OS.
Sure, in theory you could use other shells and such on top of the Linux kernel, or wrap GNU software around a different kernel, but since that's largely not done there's no confusion if you just say "Linux".
So the choice is between an unwieldy name that most people don't know and that adds nothing to the clarity of the speaker, or a simpler, already-accepted, one that's just as unambiguous. Me, I favour the latter.
*Well, technically it is, but it's such an early alpha, despite almost 30 years of development, that it really doesn't count.