Not so. The material in "Unfinished Tales" was known since at least immediately after Tolkien's death, but publication was delayed for years until "The Silmarillion" had been published and allowed to soak up bestseller attention for a while first. The same is true of "The Tale of Beren and Lúthien", which Christopher Tolkien was aware of all those years since his father's death, but it was decided to save them for a rainy day.