That definition isn't wrong. However, the "one-of-a-kind digital asset" is the NFT itself—basically a digital autograph—not a copy of the image (or book) the NF

That definition isn't wrong. However, the "one-of-a-kind digital asset" is the NFT itself—basically a digital autograph—not a copy of the image (or book) the NFT describes, or even a license to such a copy, and certainly not the copyright, unless the copyright holder explicitly agreed to that when minting the NFT. (And you would need some out-of-band proof that the NFT was truly minted by the copyright holder.)