There is no way a person in his position has any time for focus. CEOs of big companies (especially in case they manage multiple companies!) usually act as firefighters and mainly mentors to top-management. They need to be able to quickly jump between calls, board meetings, press, Q&A, in matter of minutes.

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.