Depending on how you measure it, the share of credit which comprises the USD money supply dwarfs the amount of actual cash by something like 100:1. The cause of the deflation in the 2008 crisis was a sharp contraction in the amount of that credit, and the crisis only ended when that contraction stopped, not because of the Fed's relatively much less significant cash injections.

The sorts of sharp, catastrophic deflations that took hold in 2008 (or 1929 for that matter) are always caused by the bursting of a preceding credit bubble, usually created by central bank policy. I don't see how that kind of bubble can happen in Bitcoin. So the argument that "Bitcoin is deflationary" is true as far as it goes, but that doesn't mean that a Bitcoin-based economy would necessarily look exactly like, say, the Depression-era U.S. They're two very different kinds of deflation.

In the absence of major inflows & outflows of credit, the addition of liquidity to a market usually reduces the severity and frequency of big price swings. The very difficulty of issuing credit in BTC may very well prevent it from becoming a viable currency, but I think it's also an important brake on speculation.