Maybe he did it to fire a shot across their bow?My understanding is his shares are common stock so while 9% might make him the largest single shareholder it's still no majority. Common shareholders have little influence in the actual operation and policies of an organization. He'd have to secure a seat on a board of directors to have any direct influence on Twitter's policies.
There's also a complication that Musk likes to play fast and loose with SEC rules. He waited too long to file the reporting of the share purchase so now he can look forward to the purchase being scrutinized. At best he'll probably pay a (comparatively) small fine for not filing promptly but given his track record they may decide to levy a more severe penalty including making him give up the shares like they did with his own company, Tesla.