"Sign in with..." is almost SSO. Federated authentication employs centralized authentication and can even support third-party identity providers. However, single sign-on is exactly like it sounds: You sign on once and have access to all SSO-enabled applications. This differs from "sign in with..." offerings, because you often have to sign in again. Compare this to, say, the user hub of Google Workspace: You are presented a list of applications, and when you click on one, you are instantly signed into it. This is SSO.