> The value of the cumulative labour of Mailchimp's engineers is $12 billion
No, that's the value of MailChimp as a company. That includes the product, the branding, the management and the customer base.
> The value of one's labour is the value it generates - not what anyone is willing to pay for it.
The market value of anything is exactly the value that anyone is willing to pay for it and that's the only value that actually matters in this case. No one knows the exact value that anyone specifically contributes on a large project. How much of MailChimp value is due to its engineers and how much is due to its sales team? The only thing sure is how much the company agreed to pay its emgineers and that didn't include equity.
If you want to own parts of the company you work for, go work for a company giving equities. If you work for a company which doesn't, you are not entitied to a part of the company value when it's sold. That's literally the meaning of being an employee.