That 2nd sentence is incorrect. The blockchain is already the 3rd party attesting to the NFT's legitimacy. The purpose of the decentralized blockchain is to remove the need for 3rd party validation. Yes the NFT can be copied, and yes the digital asset that the NFT represents can be copied, BUT the NFT's record in the blockchain can NOT be copied. What is meant by "own" is that the owner's address is tied to the NFT in the blockchain record.