Authentication plug-in

Question: 

Would an authentication plug-in which still uses the local database for authenticating users need to implement the IExternalAuthenticationMethod interface?

I guess I am really trying to override this function in Nop.Web.Controllers.CustomerController without changing the code there

var loginResult = _customerRegistrationService.ValidateCustomer(_customerSettings.UsernamesEnabled ? model.Username : model.Email, model.Password);

Amy insights appreciated. The doc is not really covering that topic being discussing the facebook provider.

Thanks
T