Yes I was thinking about chrome profiles. My mistake. But from what I remember, when you choose to log out, Chrome prompts you if you want to wipe your local data. So logging out locally allows you to protect some data. I think there was a way to log out remotely (tricky/weird way), though again without the local-wipe prompt.

I also just tried signing in to chrome (69.0.3497.100), and it tried to sync immediately again, so I'm not sure where the "sync is optional" idea comes from?