That doesn't mean it can't function as a currency, just that it maybe isn't a good one.
This is an important distinction, because critics have adopted this talking point of "not a currency" or "can't be a currency" as some kind of binary, but then have terrible arguments backing this up. The difference between 2% inflation and 0% inflation is just that - 2%.
Mind you - Bitcoin is currently issuing at a rate of 1.5%. The reason it is volatile has absolutely nothing to do with it's "deflationary" nature.