There is no good California case law covering your issue that I can locate. The question boils down to what were the parties' expections under the severance contract, because it could be interpreted either way. As a general proposition, where an ambiguity exists in a contract, it is interpreted against the draftsperson -- which is probably the employer in your circumstance.
So, if there was no discussion of when your pension rights would terminate, and you were continued to be paid salary and benefits after your last day of labor, then that could work in your favor.
Unfortunately, the only way to find out for certain is to sue the employer if it doesn't agree with your interpretation.
For an employment rights lawyer referral, see this link
Hope this helps.
NOTICE: My goal here is to educate others about the law. I am always available to answer your follow-up questions after you click Accept – however, if you do not click Accept, the website gets paid, and I receive nothing. This is true, even if you are on a subscription plan. So please click Accept, so that I will be able to continue to provide this service for others in the future.
And, if you need to contact me again, please put my user id on the title line of your question (“To Socrateaser”), and the system will send me an alert. Thanks!