The stock options were written to expire after ten years and the ten year expiration date was coming up. Airbnb wanted to IPO to ensure these early employees got properly rewarded for their huge contributions. I’m not sure exactly why they couldn’t just grant replacement options but I believe this would cause issues with the cost basis / additional tax and they probably can’t issue equivalent options to some of those people who no longer work for them due to IRS regulations

Do they need an ipo to exercise them? What about private sale of shares?