He can only write what he knows about though or whatever he felt he wanted to get off his chest.
Yes I agree and the book is mainly about his own journey. I just had ( and just to make it perfectly clear this is only my opinion and the way I read the book) this odd feeling that he was not totally convinced that JB was guilty at the time of the court case . He certainly did not think much of "the family" and the way he was treated by them .
Later on with the Arguments between him and Jeremy ( not getting into whether they were right or wrong) I think he was convinced - and like we are constantly being reminded he is guilty in the eyes of the law - therefore he must be guilty.