This tale is all too believable. I wish I lived in a world where the police had the common sense to ask themselves, Why would the person who collected this material bring it to us? It makes no sense whatsoever that he would.
Anyway, independent of the truth of the anecdote, the advice seems sound. Why risk it?
Chance of you being jailed for it: above average. Chance that anyone will be helped by you turning it in: far below average. Take your chances, if you dare.
Doing anything to cover yourself, such as sending it to the cops through the mail anonymously will further raise the chances that the cops will try to pin it on you.
Anyway, independent of the truth of the anecdote, the advice seems sound. Why risk it?