When people say, bartered otter furs for a donkey, were the furs a currency? Was the donkey a currency? It seems a bit of a stretch. But in any case, it was the word 'fiat' which, to me, was the biggest violation.
However what money is (the abstract concept) and what a currency is (the actual thing you use) is subjective. Art could be used as money and a currency, just as well as sea shells or cigarettes can.
The key is it being used as a medium of exchange, not as something you get for its own gain. So trading art, for the sole purpose to trade it for something else, is using it as a currency.
I think realistically modern art is more of an investment vehicle than a currency.
I'll add that art is probably used for tax avoidance and money laundering as well.