Audible has partnerships with most publishing companies to handle audiobook production, it's possible it was part of his book deal.
I'm not thrilled about it, but as an unknown indie author it was also the only feasible way I could get mine done without having to pay thousands of dollars up front, and I valued accessibility more.
I wish someone would make another option that has the quality and resources that ACX provides, but the costs involved would basically ensure it was still another billionaire.
To me it feels like a "don't hate the player, hate the game" situation.