In my opinion, the author’s note should be considered as an excuse. It basically says that the author of the book changed little bit such as using pseudonyms. I think it isn’t special to do that because non-creative writers have to protect the characters’ privacy. Additionally, readers can still enjoy reading the book even though he has done that. Since we don’t know what the characters’ actual names are, we don’t notice any incongruities. I wouldn’t care if John Berendt didn’t write the author’s note.
I just wonder why Ms. Sutcliffe made us to read it before we started to read the book. I don’t see any differences even if we would have read it after we finish the book.
Anyway, the author’s not isn’t important.