I think that is the "minting" problem.
An artist simply signing stuff does not even prove that a physical object corresponding to the token exists.
I like to think of other use cases, like real estate or stock options. If you talk about a plot of land, it seems feasible to find a unique identifier (you could even involve the government).
Edit: HN does not allow me any more replies right now, so I have to leave it at that. The reason why a "government signature on the deed" would achieve the same thing is because we trust on the government to keep a proper ledger. And the government asks for a lot of money for a transfer.
A deed digitally signed by the government's private key satisfies this need without a blockchain.
Only because the government keeps track of who owns what in a centralized database. Typically they also ask for pretty high transferal fees.
The answer is that this is not a problem, because the whole point is knowing who the creator of the NFT is, and beeple trying to create additional copies using a secret wallet but not telling anyone will be treated by the market the same as me minting a copy of a beeple piece.
The problem being described above is the possibility of the trusted author minting multiple digital signature NFTs for the same content. A reseller can't create a copy because they can't sign a message conferring ownership from the trusted source without their private key.