I use standalone Word, but it's on Windows, where I've been able to disable Copilot, so far.

Think I found an Apple Support discussion that sounds like your post! In any case, if the Copilot widget was added on Mac ahead of the Prefs > Proofing Tools check box that's meant to go with it, that's a pretty bad UX bug (atop the bad UX of Copilot itself), and I am sorry.