The amount of second hand content an LLM needs to consume to train inevitably includes copyrighted material. If they used this thread, the quotes OP included would end up in the training set.
The amount of fan forums and wikis on copy written material provide copious amounts of information about the stories and facilitate the retelling. They're right that it is impossible for a general purpose LLM.
My personal experience so far though has been that general purpose and multiple modality LLMs are less consistently useful to me than GPT4 was at launch. I think small, purpose built LLMs with trusted content providers have a better chance of success for most users, but we will see if anyone can make that work given the challenge of bringing users to the right one for the right task.