Their methodology seems reasonable to me.

To clarify, they look at the probability a model will produce a verbatim 50-token excerpt given the preceding 50 tokens. They evaluate this for all sequences in the book using a sliding window of 10 characters (NB: not tokens). Sequences from Harry Potter have substantially higher probabilities of being reproduced than sequences from less well-known books.

Whether this is "recall" is, of course, one of those tricky semantic arguments we have yet to settle when it comes to LLMs.