There have been at least a half dozen of these deals in the past 1-2 years including Google “licensing” CharacterAI to pull their founders back into Google as valued employees.

In the deal mentioned above: my guess is that preferred class shareholders and common shares got paid out but the common shareholders had such a low payout that it rounded down to zero for most employees.

This can happen even in a regular acquisition because of the equity capital stack of who gets paid first. Investors typically require a 1x liquidation preference (they get their investment back first no matter what).