I don't pretend to know the ins and outs of this, but the dividend is to common stock. What if the next step after closing the deal would be founders telling employees to exercise as much as possible, and then announce the dividend?
Granted, most of that money would probably have gone towards AMT, but let's not vilify AirBnB before we hear their end of the story.
They are not selling their stock for that $20M lump.
The only 'foul' thing here, is that their employees (option holders) will probably get nothing.