I’m always suspicious of these kinds of tests. It needs to be run with an unpublished book, not one of the most popular series in the 21st century.
If you want a real test, go test it on some Japanese light novel, or some harry potter fanfiction, and see if the model actually understands the plot details.
For reference, Opus/GPT-4 know the rough story of moderately popular light novels/mangas without any context given. They however do not precisely understand the fine-grained details of the story, like which character will win in a fight.
But those Japanese light novels don't have millions of forum discussions and essays written on it. So it shows how well the model can recall sparse data in its training dataset, rather than recalling a dataset that basically shows up 100000 times in different forms.