This could be used as evidence of their authorship in court.
“Look, here we have a tamper proof ledger showing I uploaded this at X date before it was copied.”
There are other things people could consider though, in terms of evaluating the history/authorship of something. The blockchain is just one data point.
Eg, I recently saw the “Charlie bit my finger” family will sell an NFT of their video. Is there anything stopping someone else from selling NFT’s of other viral videos before their makers do so?
Timestamping would only work for works that are tokenised at the same time as (or prior to) their public release.
If there is other evidence, e.g. the other party has a YouTube account which uploaded a video years ago of the work, and is publicly known to be the author of the work, then the judge will probably rule the NFT is not sufficient evidence of ownership/you are not the copyright holder.
There is nothing technically stopping people uploading NFTs of things that they don’t own the rights too. That won’t hold up in court though, not even the court of public opinion.