The data doesn't support this. RescueTime did a study on the time that their YCombinator class spent "actually working" (their software tracks which window on your computer screen has focus, and how much time you spend interacting with it):
http://news.ycombinator.com/item?id=209195
Barely anybody hit 10 hour days of solid productivity; typically, people can manage about 7ish hours of actual work, only 3-4 of which is coding or other heavily intellectually demanding tasks. This squares with several worker-productivity studies done around WW2 (when major corporations were deciding whether to move to the 8-hour workday; the standard at the turn of the century was 14 hours), which found that productivity decreases after 8 hours, and the time spent dealing with mistakes eats up any marginal increase in output. Having worked in one startup where much of the eng team seemed to work 12-14 hour days, I can anecdotally confirm that.
It's not that they're dicking around, it's more that they are probably staring at their computer screen cursing or berating themselves. Or feeling despondent and miserable about their chance of ever shipping product. Or writing code, but writing the wrong code which will just have to be ripped out once they think about the product again.