This is corporate trolling. Open AI is trying to go public soon, and Elon did this to fuck with that.
Massive oversimplification but basically: In the United States, you are required to take the best offer when selling a company, unless you can argue in court that you had reasons not to sell to whomever placed the highest bid. Because of this offer, as open AI attempts to go public, they will have to either to be close to Elon's bid, forced to sell to him, go to court and fight him ther, or be forced to remain private.