Pretty much. They are hoping you don't know the law. They are also hoping that by encumbering you with one that a future poacher would know that they might have to spend money on lawyers to fight for you in case you get sued, even though they know you will win.

Is this to imply that non-poaching agreements (e.g., "Great, you quit, don't talk to my employees") are equally void?

While IANAL, yes, non-solicitation clauses are also not enforceable in California.