No its not. What you miss is that the creation of the signature is stored in the blockchain. So the creator can proof, that the signature existed before the oth

No its not. What you miss is that the creation of the signature is stored in the blockchain. So the creator can proof, that the signature existed before the other signatures. If you modify a video, your signature would have a timestamp AFTER the first signature. Proofing that this is a modified version of the original content. Going a little bit further, modified versions of videos could be signed by the private key of the creator, proofing that the modified version was published with the consent of the creator.