However, as soon a detection tool becomes publicly available (or even just the knowledge that watermarking has been implemented internally), a simple enough garbling LLM would pop up that would only need to be smart enough to change words and phrasing here and there.

Of course these garbling LLMs could have a watermark of their own... So it might turn out to be a kind of cat-and-mouse game but with strong bias towards the mouse, as FOSS versions of garblers would be created or people would actually do some work manually, and make the changes by hand.