It's not that it's a more or less 'appropriate term'.  Correct me if I'm wrong but I think you're thinking of 'inflation' in the way it's used by politicians and the papers rather than as an economic technical term.  There are two distinct meanings.  The more common usage is measured relatively to what can be purchased with a currency. e.g. a how much a set 'typical grocery basket'  costs to buy a year apart.  In that sense of course Bitcoin is deflationary.  But the technical use is to do with supply money supply.  Bitcoin is only deflationary from a technical perspective if more coin is being lost than is being introduced which is very unlikely to be the case.  In terms of 'market cap' measured in USD it can only be higher with Bitcoin at $20 than it was at $30 if there are > 50% more Bitcoin in existence i.e. its supply has inflated by 50%, which is how thefiniteidea is using it above.

For fun I posited in another thread that relative to total supply even the rate at which Bernanke's extreme USD inflationary policy is being implemented, as a monthly rate he's only introducing approximately half as much new currency as Bitcoin is.  Of course this doesn't tell the whole story because for BTC we know from Day 1 how the monetary supply will pan out but despite the popular notion otherwise, for the next few years the rate at which new coin is introduced makes Bitcoin a heavily inflationary currency