That doesn't make sense so I went and looked it up.

According to the S-1 filing of SpaceX, shareholders (which include employees with stock) can sell up to 20% of their stock starting on the second full day of trading after the company releases its first earnings report.

So each employee can sell 20% of their stock (probably options, but since they've had an "Employee Share Purchase Plan" (ESPP) some will hold actual shares). If 80% of someone's holdings are dependent on them staying at SpaceX I doubt many will leave after selling 20%.