User blog comment:Menara/The Son of Sobek: The official PJ and Kane Chronicles Crossover/@comment-174.89.111.236-20130516230425/@comment-3146930-20130516235807

Well not all souls seem useful. I mean Echo and Narcissus managed to escape the Underworld, but don't really seem useful to Gaea. As far as the above comment, Percy could have thought that Carter used the Doors of Death to escape the Underworld before Thanatos was freed or even before they were closed. He just hadn't been captured or defeated yet.