I thought a factory would be more illustrative here. Typically you would provide a Storage and Adapter to the AuthenticationService for it to be useful.?
Should I extend the factory to make this clear? Because you're right that at the moment it does the same thing as setting an invokable.

Also I wanted to show that if you're using something like ZfcUser, you would only have to alias 'zfcuser_auth_service' and be done.