There's no way he would ever admit this, but I bet one of his motivations for posting that tweet was to move the stock and hurt the shorts.
Musk seems to be one of those classic "force of nature" founders, whose will to overcome obstacles and prove everyone wrong is essential to getting off the ground, but could be a liability in a mature company.
"Oh, yeah? Try and stop me!" works great until you run up against someone who can actually stop you (often, the government).