Mostly lactic acid. I suspect their entire brewing process from mash to fermenting lends itself towards the more acidic beer but a really important part of their process is the yeast and bacteria. They aren't pitching a White Labs or Wyeast blend. The wild bacteria they use probably has greater acid production and/or greater tolerance for alcohol production than what we get from the labs. The labs produce blends intended to be viable and produce a particular flavor profile. That means giving up some of the more aggressive strains to make sure when you pitch one of their blends you get repeatable results without having to spend decades developing a consistent and enjoyable flavor profile. So the use of dregs out of some of those very sour gueuzes would probably help, especially if you could rely upon them for souring without having to add a lab-created blend.
Another thing I didn't think about before is using staged pitching rather than adding blends. That's what Russian River does. They ferment the beer out, then add bacteria, then add brett then blend from there. So rather than buying a $7 blend you might want to buy the separate lacto, pedio and brett cultures and pitch them in stages in this manner, although that seems a lot more expensive than using dregs.
I mentioned the acetic acid because Americans tend to like their sours more acetic and a lot of American sour brewers look at increasing acetic acid production to get that very sharp acidic bite. I seem to think I heard Vinnie say that in an interview but it might have been somewhere else. However, that may not be the particular flavor profile you are looking for. I know I am not the biggest fan of acetic beers.
I also believe ryane has talked about keeping some very sour beer he uses for blending. He might have talked about it on his blog or in posts here. I think his blog is ryanbrews.blogspot.com.
