Apologies if this wasn't clear but these are California-centric offers (and can probably be extended to include Seattle and New York). They are not across the US, or across all software jobs.
Same way how .io is not exclusively the British Indian Ocean Territory ?
To be clear, anything you directly contributed from your paycheck is yours and isn't subject to vesting. That's always your money. The part that may have vesting rules is the employer contribution.