Obviously it depends how you define 'currency'. Dictionary.com has at it's first meaning "something that is used as a medium of exchange" as it's first definition, for which bitcoin qualifies. From that point of view it is in some ways better than USD as there is less friction for international transfers.
In other ways it's different - most currencies are backed and regulated by governments. Though things like cigarettes have been used as currency in prisons. I guess the test is whether you can go buy stuff with the thing in which case bitcoin qualifies while gold does not.
The question of what units you use to price future transactions is interesting and more what Buffett is talking about. Like if I'm going to offer to sell you some oil next month I'll probably price it in $usd as that's conventional, and almost certainly not in bitcoin as who knows what it'll be worth in a months time. You could argue though that we could do with some other unit to price contracts, say something like average annual US wage divided by something, that would not be change value much with inflation in the way that fiat currencies do, so you could use it for say pension payments and say in 50 years we'll pay 0.5 average wages or some such which is kind of what you want rather than saying us$40k - who knows what that will buy in 50 years.
In other ways it's different - most currencies are backed and regulated by governments. Though things like cigarettes have been used as currency in prisons. I guess the test is whether you can go buy stuff with the thing in which case bitcoin qualifies while gold does not.
The question of what units you use to price future transactions is interesting and more what Buffett is talking about. Like if I'm going to offer to sell you some oil next month I'll probably price it in $usd as that's conventional, and almost certainly not in bitcoin as who knows what it'll be worth in a months time. You could argue though that we could do with some other unit to price contracts, say something like average annual US wage divided by something, that would not be change value much with inflation in the way that fiat currencies do, so you could use it for say pension payments and say in 50 years we'll pay 0.5 average wages or some such which is kind of what you want rather than saying us$40k - who knows what that will buy in 50 years.