(Uninvolved growth-stage CFO perspective)
The IRS mandates that stock option grants expire after 10 years. My best guess is these early employees are quickly approaching those grants' 10 year mark, and face an exercise or "lose it" situation.
If you exercise, you have to pay the gain. For early employees, this could/would be a massive bill -- probably well into the 7-8 digit range for some early hires.
Stripe seems to be teeing up a secondary sale of these stocks themselves -- where Stripe is offering to buy some of their shares back from those early employees, allowing the employees to exercise ('buy") all of those early options and (at least) pay their tax bill.
Usually, a company will limit the # of shares eligible for a secondary purchase so current/former employees can buy all the stock, sell enough to the company to cover that person's resulting tax bill, and keep the remaining stock until IPO. By limiting the # of shares they will allow to be purchased via the secondary, Stripe will almost certainly make sure the secondary does not create post-tax cash gains to make people wealthy!
It doesn't have to be that way:
Stripe could allow ANY investor to buy the shares directly from those employees -- but guessing Stripe has a blocking right on stock transfers-- so they have, and will continue to block early employee sales to new investors. Stripe and many companies have these transfer "veto right" to make sure they can control their ownership ("cap table") and also to make sure early employees don't get rich before the IPO.
This is my best guess-- I am not familiar with the Stripe's situation.