What I think it means is that APART from these instances, the characters in LOTR refer to things that were already part of the wider creation of Middle Earth - for example Gil Galad , Beren and Luthien, Earendil already existed (in the unpublished Silmarillion) as did Gondolin when Tolkien referred to it in the Hobbit. Tolkien had been working on Middle Earth for decades before he started the LOTR and the characters that know some of the history of Middle Earth refer to it . However when Aragorn refers to the cats of Queen Beruthiel and Saruman (Gandalf ?) to Five wizards it is the rare occasion when the character "knows" something that the author doesn't. They are referring to things that Tolkien hasn't already fitted into his creation. Tolkien later did write up a history for the Cats and the Wizards. Not sure if that makes sense but really it is a question of order. He made the story to fit the reference rather than making a reference to a story he had already written.
