No, its not really necessary that you have been told to be the Miller which is a theory that ist just as believeable as Haggers claim.
At such a late time in the game maybe Hagrid thought it was time to let us know his investigations because it would be a loss not to know any of his results before he dies. With also many Mafia dead it is thinkable that there´s a chance of just one ore two remaining and Rubeus is acting really selfless.