This excerpted anecdote indicates it was mainly a leadership gesture to rally the troops. Still pretty cool that he'd bother, though.
Why would he stop writing code? I can understand he might be too busy (and it might be getting too advanced?) but I code because I like it, not because it one day might allow me to make money. I do not plan on stopping whatever happens.
Because he was never a particularly talented coder, and because FB now has some top-tier talent that can do a far better job at coding that he can. Besides, contrary to popular belief, running a $100 billion company probably doesn't leave you with much time.