“Transformative” in this context does not mean simply not identical to the source material. It has to serve a different purpose and to provide additional value that cannot be derived from the original.
The summary that they talk about in the article is a bad example for a lawsuit because it is indeed transformative. A summary provides a different sort of value than the original work. However if the same LLM writes a book based on the books used as training data, then it is definitely not an open and shut case whether this is transformative.