I'll post my response from Jean's thread in this thread for those who don't keep up with that thread.
If I may ask, what explanation would be more "concrete and definitive" than "You are the Phoenix, now and forever"? It seems some fans—and this is not aimed at you specifically—wanted or expected Jean to turn into Phoenix in this issue, or for the Progenitor to explain the intricacies of Jean's identity as Phoenix or even White Phoenix, even though such an identity—such a dynamic—is far beyond a Celestial's purview and perception.