One possibility is that for a startup founder, there usually is a great variety of work.
One hour you could be writing code, the next answering a support email, the next pitching a prospective customer, and so on. It could be that by switching gears every few hours, people are able to retain some of their productivity.
Context switching is, in the literature, known to be productivity destroying, and while it may feel like a very productive day if I do things from four categories, I can tell you that when I actually go back and look at the results it is feelings-nothing-more-than-feelings.
e.g. If I were doing nothing but customer contact mode, I know that I can send 50 emails in a day of moderate complexity. (e.g. Consulting mini-proposal for a client, support issue for BCC customer, pre-sales question for AR, ... x50. I know this because I have verifiably done it on e.g. launch days before.)
I did four things today -- video recording, hopped on a call, sent emails, and... shoot, only three things but I thought I remembered four. My subjective experience is that 1/3rd of my day was email. How many emails did I send? 1/3 * 50 = ~15? No. Five. And it (subjectively) felt like pulling teeth even though three of those were the same email with a few hand-written edits in the middle of them, whereas after I get into email flow mode for I can normally do two hours without getting bored.
I've got at least two hours into working with video today. How many 5 minute videos have I completed? Umm, zero.
Contrast this with yesterday: yesterday I putzed around to 6 PM and then coded until 10 PM then quit for dinner. I got two features coded and tested then squashed a pair of bugs. Yesterday feels non-productive in some ineffable way ("I putzed around until 6 PM! I only worked 4 hours and my salaryman brain rebels against the sheer laxity! All I did was code six measly little commits!") but it almost certainly added more to the business.