Long gamma gives them hefty tailwinds when volatility rises, but it's not the whole story.
IBKR didn't do too well in Q2 2010, when the VIX took a trajectory much like this past quarter. In Q3 2008, market-making volume actually went *DOWN* (though they had a sizeable boost to profits).
However, other indicators are also favorable. Over the past year, competing market-makers have been pulling out, so Timber Hill has been picking up share. I'm also seeing wider spreads on options that I trade (but I don't know if the situation is the same across the market).