I read the $850m being well after acquisition. It was an attempt at hush money, and he didn’t sign on. He’s a multi-billionaire, and values being able to speak opening over a bit more money.
In support of that point, from the article: "He also reveals that when he left the company, he declined an offere [sic] of $850,000,000 in unvested stock in exchange for signing a nondisclosure agreement that would have kept him from discussing any of this."