"their are authentic way to sing traditional songs"

Not sure about that. An elegant tenor would adopt a different approach to the material than a bass surely?

Thomas Hardy talks about a character singing like 'a bee in a flue' (in Return of the Native IIRC) so it's safe to assume certain modes of presentation were cliched 150 years ago. One of the more resistant factors about folk music becoming more widespread could be the mannered approach in which it is sung. I'd have thought any voice that can hold a room unaccompanied and without amplification is a potential folk voice, not the accent or mannerisms.