I think this is rather an accurate assessment of Holden and the impact that Phoebe has on him. One indicator that Holden lives in a world of his own chosing is that he considers everything he doesn't like or agree with to be "phony." What shows that he is to subjective is that he ignores the way that his own character is "phony" and inconsistent. What is interesting about the role of Phoebe is that she seems in someways to be the opposite to Holden, as she loves life and takes great enjoyment in it. In fact, Phoebe tells her brother off for his disenchantment with life and the way that he is unable to enjoy it. Phoebe, therefore in a sense helps act as a catalyst to bring Holden to a point of crisis, as he immensely values the opinion of Phoebe, and considers her to be the only person who can understand him and with whom he can communicate. She, according to her brother, is the one non-phony character in the book.
Let us also remember that it is the sight of Phoebe riding the carousel in the penultimate chapter that makes Holden, for the first time in the novel, ecstatically happy and brings him to the point of breakdown. Therefore, it is entirely accurate to say that Phoebe acts as a catalyst upon the character of her brother.