IANAL but severance isn't required by California law so I'd think that if their employment contract promises some, the situation completely depends on what else

IANAL but severance isn't required by California law so I'd think that if their employment contract promises some, the situation completely depends on what else the contract has to say about conditions where it can be taken away. "Severance offers" in my experience are a new contract they throw at you at the end, they offer some cash and maybe benefits awhile and you have to make some new promises in return.