While i do believe there are people out there who are not motivated by money, to me claiming "..money doesn’t make much of a difference... most of it’s gone. I’m kind of glad it is." smells of cognitive dissonance.
[1] http://www.wired.com/epicenter/2012/04/facebook-buys-instagr...
So let's say he was hired. Maybe his engineering decisions send Instagram on another trajectory - one that doesn't involve a billion dollar exit. Maybe his impact on candidate interviews causes Instagram to end up with an entirely different-looking engineering team, one that isn't as successful. Maybe he develops considerable influence within the organization, and convinces the founders to take an earlier exit.
This isn't a knock against Robert. This is just suggesting that things could have played out entirely differently were he onboard - even if he was by every measure a successful employee.