It is an NFT because:

A non-fungible token (NFT) is a unique digital identifier that cannot be copied, substituted, or subdivided, that is recorded in a blockchain, and that is used to certify authenticity and ownership.[1] The ownership of an NFT is recorded in the blockchain and can be transferred by the owner, allowing NFTs to be sold and traded. NFTs can be created by anybody, and require few or no coding skills to create.[2] NFTs typically contain references to digital files such as photos, videos, and audio. (source: Wikipedia - Non-fungible token)

In our case the NFT references to the previous Bitcoin address.

It is not an altcoin because NFTs are uniquely identifiable assets, they differ from cryptocoins, which are fungible.

But you should consider that 'signature chain' is a new type of NFT. Read the second post to understand how it works.