Apparently, this is an inconsistent, I believe there are many like this lying in Java EE API doc and other specification docs like EJB Core.
There is a compatibility note at 4.3.5 The Optional SessionBean Interface. I think they did not bother updating the API. I agree that it is inconsistent, but if you want to make a session bean, you look at the spec first, not at the API. Then you know that you don't need to implement this interface.