Butcoin was supposed to be money, but so far it's far to volatile to be used as a unit of account in any serious sense.
Using it as a a unit of account is a regulatory definition, not an economic one. Money is still money even if there is no concept of "Bookkeeping."
Volatility doesn't enter in to the equation, during the Weimar republic the Mark was still the currency of Germany, even though hyperinflation made it essentially worthless. Just because it isn't a good store of value doesn't mean it isn't a store of value. What matters is it's ultimate utility to the users.