No, what happened was that early employees got equity grants that seemed reasonable when they were hired. When Zynga became a smash hit, those unvested options became very valuable.
Then, Zygna's board decided to try and weasel out of paying the options. They made up the excuse that the employees were "underperforming", but really they just didn't want to give some low-ranking employee the $1M+ payday they had earned by being an early employee of a successful startup.
Instead of outright firing the employee (a downside of "at-will employment"), they instead pressured the employees to give back their unvested options.
That breaks the startup social contract. If the company is going to try and weasel of its options when they are a unicorn, why would any employee accept equity as compensation?
I.e., you hire an employee and give them $50k of options. Suppose the business is a 1000x unicorn, and those options are now worth $50M. Why pay a software engineer $50M? So you ask the employee to give back his unvested options, because his "fair share" was only $50k and not $50M.
Also remember that, if the employee is fired, they have 60 days to exercise, come up with the strike price, pay a huge tax bill, for shares that may not be liquid (or ever worth anything). So the employer does have leverage over the employee, because they're forced to exercise and pay big $$ if they're fired. It's unethical to exploit that leverage.