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 longer the official currency of Zimbabwe. Now that Zimbabwe officially accepts relatively stable foreign currencies, prices of goods have stabilised in that country.
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 longer the official currency of Zimbabwe. Now that Zimbabwe officially accepts relatively stable foreign currencies, prices of goods have stabilised in that country.