So they are steam keys, verified by blockchain, but just numbers associated with a piece of art. Sounds like DRM to me. Fungible doesn't mean identical. Every US 100$ bill has a unique number attached. The bills are still fungible. The numbers are not identical but serve the same functional purpose, so they too are fungible. Nobody cares what number is on their money. One bill/number can be swapped for another without impacting any function. Fungibility.
"able to replace or be replaced by another identical item; mutually interchangeable." Oxford law.
Though the DRM analogy may work in applications that interact with NFTs. For example, games with unique items. The application would limit the full experience of a tokenized object to the owner alone. For example, only n00b5lay3r may equip the Golden Sword of Evisceration, because it's in their wallet.