His argument (that articles priced in bitcoin change their priced based on the value of a fiat currency such as USD) is valid as a qualitative judgement of bitcoin's utility as a currency, but isn't really a valid test of what is or isn't a currency. Case in point: The Zimbabwean dollar remained a currency through its hyperinflation period, and is still considered a currency (however useless) even though it is no lon…
Also, if your defence of Bitcoin as a currency is to compare it to the Zimbabwean Dollar (a currency ridiculed and later abandoned) then is it OK if I ever need a lawyer that I pass on you and go to the next guy? ;-)