Just because something is volatile doesn't mean we can make no useful statements about its trend. If you're re-evaluating your purchasing and investing activity on a day-to-day basis then yes, the day-to-day price level change is relevant. (There are corporate treasurers who worry about 30-day price level changes.)
> I just can't see that as being a useful definition.
I agree. For an asset as volatile as Bitcoin, the terms "inflationary" and "deflationary" are largely meaningless. It won't be used as a currency because it's volatile, not because it's deflating.
Still, between "inflationary" and "deflationary" Bitcoin has (a) spent more days deflating than inflating and (b) deflated over its long run to date. It is more correct to say Bitcoin is deflationary than anything else.