Fungibility is subjective. Some things have characteristics that make it harder to tell individual units apart, and thus people will be more likely to treat them as fungible.

I don't know what to tell you other than what I've told you. You can have your definition of fungibility if you want, but it's not the one that has force in the actual universe. The very nature of money itself is a social convention in regard to how other people will interpret some symbolic construct. The subjectivity of that interpretation is the most real thing that there is as far as these things are concerned. Mises made this argument as forcefully as anyone could desire.

The fact that I can give you ten bucks and you'll do ten bucks worth of stuff for me is nothing other than a mutual point of view of what kind of behavior that ten-note ought to elicit. If you have preferences about which note you receive, then the notes are not fungible from your point of view. If everyone in the United States shares your point of view, the notes are, for all practical purposes, no longer fungible. If some entity can exert an overwhelming force about what our collective points-of-view ought to be, then it can often get its preference about what is equivalent to what.

The NFT example is useful because it shows how a protocol can support fungibility directly -- it can make a protocol-level distinction that X is not equivalent to Y. But protocol-level distinctions are not the only distinctions that matter, as various legal actions have demonstrated.

If I delete the .png files from a NFT collection they are just a regular token like bitcoins