I’m super confused about this too, there’s tons of open source apps that use Sign in with Google. Shoot, I’ve used the SDK myself in little apps and never saw this.
"Sign in with Google" is just a button that apps are required to use when they allow the user to interact with Google in some capacity, but don't have their own account management system. Most apps with that button don't have the ability to get any data from Google other than your name and email address though; for apps where you are granting OAuth access to your Gmail, it is very obvious that you are doing so, so it's not likely that you have done it accidentally in the past. (Although you can obviously check on myaccount.google.com)
