For example, at one point in my life I worked 8-hour office days and 8-hour evenings as a bartender. I was astonished to discover I could do this for months without getting exhausted at all, because they used completely different parts of my brains, with virtually zero overlap. Office work was intellectual, bar work was physical and social.
I personally can't do truly effective, intellectual coding for more than about four hours in a day. After that, it exhausts me. But I can happily deal with meetings, or project management, or marketing or whatnot. So because startup founders can often divide their time up into blocks of entirely different tasks, it makes sense they can work effectively over longer hours.
Note, I'm not talking about being interrupted all the time, though -- I'm talking about 4 hours of coding, 4 hours of meeting, 2 hours of product, 4 hours of marketing.