By authentic I think you mean original, yeah, and without timestamp attestation (like opentimestamp) you can't provably claim that you are the first to say it.
Discussion
No, I mean, the message could always be a copy. Timestamp attestation will allow you to claim an event at a certain moment in time, but you're attesting the message-carrier, not the message. The message could be a duplicate where the original is 100s of years old.
Yes, you can also claim to be the originator and have a form of proof that others won't have. The 100 year old original cannot make a provable claim to being original. It has to be carbon dated or whatever but you have to trust whoever did the carbon dating to not be lying. I think this gets important as the ability to memory hole and change facts to support new narratives Orwell-style becomes easier in the digital age.