This is indeed impressive, I personally haven't accomplished anywhere near as much in the same timeframe. This is great PR here, and this is what would have been accomplished with a relatively meager spend on A PUBLIC RELATIONS DEPARTMENT at Tesla. A steady trickle of undeniable Good News would have gone a long way and made for a smoother road for stockholders as well as made The Shorts lives a little harder. The value of one good Tweet (X-ing? what are we calling these now?) is one thing, the value of a steady stream of Undeniable Accomplishments over the same time frame would have been a lot more.
Regarding the stock, the same basic tools Elon uses to spook the market could easily be turned to stoke the market. Hiring a professional is expensive for sure, but its often cheaper than the alternative. This one tweet won't garner the attention or hold the weight theses accomplishments deserve.