Yeah, he had to pay an 11 BILLION dollar tax bill to California and the IRS, for stock options issued in 2012. It was reported as the largest federal tax bill in history.
If he didn't sell the stocks then he wouldn't have to pay any tax on unrealized gains. According to him, he sold the stocks so they he would pay taxes in response to criticism of not paying his "fair share" in taxes.
Even if you think he'll have to pay that which from this tweet it seems he's going to try to dodge, he got to sell a lot of stock without anyone thinking he has any doubts on Tesla, which would cost a lot more than 1 billion