IIRC on my platform, when they added the feature they turned it on by default, as an auto-installed update.
And if you're logged into the gmail app on the same device that also logs you into authenticator.
You didn't do anything wrong.
Not true. See https://news.ycombinator.com/item?id=42471459
I don't particularly like that my codes were apparently synced to Google's cloud without my being aware, or the ux that prevented me from noticing. But I'm pretty confident that, having disabled the cloud sync, Google no longer has my codes
(And in fact I verified this by installing the authenticator on a tablet before turning off sync on my phone. The codes vanished from the tablet.)
In principle, yes I should rotate all the secrets. Because google may have borked their data retention, or is just outright lying and keeping my secrets. In practice, though, for my personal account, I'm content that nothing has been compromised.
Based on just your intuition. Since you don't have access to the backend specs or code, assuming this isn't a responsible security practice. It is a shortcut you can choose to take personally but should never take with any professional credentials.
I'm going to point out that you responded "Not true." instead of adding a caveat about how you personally choose to ignore security best practices for personal accounts.
I could have been clearer, but that was in response to the asserion of "you can't revert".
To answer my own question: tap the profile pic (top right on Android) and choose the Use Without an Account option. Removes codes from cloud storage and any _other_ devices. Mentioned in TFA.