You're right that options are granted at the 409a price (the common stock price) instead of the price investors pay in the financing, but as companies become more mature, the common and preferred prices usually begin to converge somewhat. In a seed financing you might see common stock valued at 10% of the preferred price, but in a late stage, billion dollar company there will be much less of a discount applied.

Employees granted options recently after the highest valuation financing would probably be completely underwater if the company is really worth $2b.

Common hasn't been close to 10% in a loong while - common is likely presently valued at >75% of Preferred. Anything 1/9 to 1/11 is strictly seed-level discounts for common to preferred. Dropbox may even be at a point where it's no longer options but RSUs.

When common is equal to prefererd, you have a public stock price.