I am not a lawyer and, not having read the Musks’ post-nup, I have no opinion about whether or not it is valid.
However, every real-world legal system since the beginning of time has put limits on what kinds of signed agreements between two people can be enforced as contracts under the law, and everyone who has run a multimillion-dollar businesses is aware of this. If the contract that Mrs. Musk signed was unconscionably unfair to her, Mr. Musk probably should not have drafted it in the first place.
However, every real-world legal system since the beginning of time has put limits on what kinds of signed agreements between two people can be enforced as contracts under the law, and everyone who has run a multimillion-dollar businesses is aware of this. If the contract that Mrs. Musk signed was unconscionably unfair to her, Mr. Musk probably should not have drafted it in the first place.