Is he working though?? It seems to me that he just spends all day on twitter. Not all that different from myself actually, except no one is writing books and stories about me and my mythical 100 hour work weeks.
Something very similar is also written in Cal Newport's Deep Work book, where Jack Dorsey's typical day is outlined just as a series of interruptions.
I guess that tweeting between these interactions is not that time-consuming for him, + he actually sleeps at the workplace and is 100% focused on work.