I suspect they used a pneumatic medium-crown stapler. wborst's link is one example. They're not all that hard to find, although nowhere near as common as narrow crown pneumatic staplers.
I can't think of an obvious reason why a narrow crown staple would not work adequately if you used more of them. I do not speak from experience here, however.
Harbor Freight does sell a pneumatic "T50" sized stapler that conceivably might have the power to do the job, as well. I've never tried one in person. I do have one of their pin nailers and so far it has worked very nicely, although the safety is rather dubious.
