The transfer does not include the private key for the address, so how can it be claimed that ownership of the address has been transferred? Furthermore, even if the private key is also transmitted, then ownership is only shared and not transferred because the previous owner still has the private key.
In this 'NFT' project we call it 'ownership'.
I think I understand completely.
Yes, you got it

My question is what does "ownership" in this system mean? What is the significance of "ownership" beyond simply controlling the address in the last signed message in the chain? How does it relate to the real world? How does it relate to the common definition of the word "ownership"?
'ownership' = to be the owner of the 'signature chain' that started with a certain address
And if this certain address is for example 12cbQLTFMXRnSzktFkuoG3eHoMeFtpTu3S (Satoshi's first transfer address) or the 'pizza address' it could have a value to be the owner of the signature chain. We have to test it.