With this feature, new users will be created in WordPress at the time of SSO that is If the users do not already exist in WordPress, the Auto Register users feature will create them automatically after performing Single Sign-On (SSO) with their OAuth Provider credentials. (The plugin does this by default, you don't need to configure any settings to make it work.)
If you want your existing WordPress site users only to perform SSO on the site, then you can turn this feature off. Using the Sign In Settings Tab -> Advanced Settings, you can turn off (uncheck) the option "Auto register Users".
Eg: You can see in the following image the user name john is not present on my WordPress site.
But after performing SSO on my site, it has been created.