My read was that Garry Tan implied "you sacrificed a lot of money in order to grandstand". I felt that was a knee-jerk dismissal of a founding employee's legitimate concern.

I'm not sure, but my interpretation is that Gary is implying that Prem Qu Nair received $20 million from the deal, and that by posting this tweet, he has violated the terms of the agreement, which generally have non disparagement clauses, and Gary will see to it that he won't receive anything.