He's a board member, and will continue to be after the sale. He's not selling because he lacks faith in the direction of the company. He's selling because he needs the cash, and as someone who made most of his fortune off of Twitter, most of his money is tied up in Twitter stock. As a board member[0] and large shareholder, he needs to make a public filing along with that sale.
If he were selling 80% of his stock, or if he were announcing his intention to step down from the board as well, I might agree with you (but also might point you to Bill Gates, who announced his decision to fully divest from Microsoft a while back, in order to fund his foundation's work). But he's selling less than a third of what he owns, and he's clearly remaining involved with the company in a capacity that bears a fiduciary responsibility.
[0] This really cannot be overstated; board members are not merely shareholders, and it's not a liability-free position. If Williams were seriously pessimistic about the future of Twitter as a board member, selling 30% of his shares and doing nothing else would be the most nonsensical way to act on that pessimism.