Well if there is written material (like the Harry Potter books) then it is considered history, not archeology. And the historians would be able to read the book

Well if there is written material (like the Harry Potter books) then it is considered history, not archeology. And the historians would be able to read the book and consider if it was treated as fiction or a religious text at the time, especially by comparing to other fiction books and religious texts which would also be available. While not every historical culture had a clear distinction between the two, our culture have a concept of a "novel" which is distinct from both history and theology. A historian looking at books from our time would be able to understand that.