ars obviously wasn't thinking critically in that comment. You're absolutely right: his proposed definition of "currency" isn't useful at all, because it would preclude talking about "the stability of a currency."

A currency is simply a medium of exchange that is widely recognized, where "widely" is subjective. Most modern currency also is notable because it has no intrinsic value (gold coins, for example, have intrinsic value). I think bitcoin certainly qualifies as a currency, albeit an unstable one.