> Fungibility means that a given asset is legally identical to all other instances of the same thing.

No, it means it is practically identical (that is, for any potential exchange partner in the marketplace, any unit of the thing is indistinguishable in trade from any other unit.) Being legally identical is a powerful aspect of practical equivalence for anything primarily exchanged in legal markets, but not the whole of it, and pretty much irrelevant to trade in illegal markets (e.g., items which are legally in the same category of contraband and undifferentiated in law may be very distinguishable and different to the people trading them.)