Alternative viewpoint: Christie may already be on his way out or on his way to becoming a lame duck. Musk clearly sees no future for Tesla as long as Christie's admin is calling the shots.
Remember, this letter is to "the people of new jersey" and not "the current governor of new jersey". Musk is choosing his venue, the court of public opinion - an arena where Christie is taking some major hits.
Progress and expansion isn't built with "expectations", it's built by repeatedly working on opportunities and winning where it was wasn't thought possible before.