[mariusor]

But at least Google can be used as a SAML idP for external services, which is what I think the author meant.

SAML as far as I know doesn't specify how exactly an identity provider authenticates a user but only how, once a user is authenticated, the user has a specific "identity" in the context of the service provider that initiated the authorization/authentication process. Therefore the authentication mechanism on Google/Facebook's side can be OAuth or something else, but once completed, the mechanism to convey the identity of the user to the originating service is SAML.