I think that's a more complicated explanation than necessary. If their private valuation is higher than their public valuation it means they can raise money more cheaply while private.
The opportunity cost of this restriction on their lives is huge.
Had they gone public two years ago, employees would have benefitted from a market of a lifetime, with equity in one of the best tickets in town.
A lot of life changing early retirements and "Fat FIRE".
What restriction, exactly?
Or to phrase parent's point differently: by not IPO'ing, Stripe forfeited the premium public markets would have been willing to pay Stripe employees for their stock.