Part of that was sunk costs. Presumably he was gambling on Powerset being a much higher exit. This store and the "I left $300k for github" are two sides to the question of what do you do when you are facing a soft landing rather than launching into orbit.

If the bonus had come in the form of a more lucrative cash buyout, it likely would have ended up being short term capital gains as options were cashed out, so there isn't even a tax difference.