It is self evident,, if Gunther was contacted by a pair of hoaxers then the book tells us nothing about Cooper. But, if Gunther was contacted the real Cooper it gives us clues about his identity.
Jude's (DanCooperHimself) video gives us insight on the technical side of writing but it doesn't move the needle either way.. We already know Gunther was an accomplished writer... but Jude seemed to miss the premise of Gunther's book,,
He was not claiming that Clara's story was factual.. or even that he was contacted by the real Cooper..
He gave his first hand experience with the Cooper contact. (His truth)
He also gave his first hand contact experience with Clara and Himmlesbach. (His truth)
And he gave Clara's narrative as told to him.. (Clara's truth)
IMO, Gunther went through this experience and could not determine if it was the real Cooper or not. He went to the FBI and they rejected it outright,, Cooper was a hoaxer.. Gunther was not convinced. So, Gunther decided to write a book to publish his experience and let the reader decide. Gunther was really expressing his own battle with trying to determine if it was the real Cooper.. and if it was then there are real consequences for the case.