Yeah there's a usability issue for sure, but the article is talking about privacy and potential GDPR problems.

Edit: what I mean is, there's no way to get the interface into a state where it looks like you're not logged in, but it is syncing in the background. The only confusion here is in the other direction: you're visibly logged in, but it's not syncing.

I was talking about privacy too, not usability... it's a privacy issue when your data is being synced when you think it isn't. (I think you might be mixing up the two scenarios?)