User blog comment:Menara/The Son of Sobek: The official PJ and Kane Chronicles Crossover/@comment-4727289-20130609195700/@comment-3146930-20130621234817

It isn't that he accuses Carter of being an escaped soul, it is that he accuses Carter of being a soul that escaped using the Doors of Death, something that wasn't really a thing until The Lost Hero, when Jason, Piper, and Leo discover the souls of deceased people returning. Percy didn't even learn about it until The Son of Neptune, six months later.