False.
> You are principally motivated by wanting to make the world a better, safer place.
Probably true.
That's not contradictory. And neither is wanting to disclose a vulnerability responsibly, wanting Facebook to be secure against the vulnerability you just found (even only because it affects you, your friends, family, people you care about) and at the same time feeling that $4,500 is a bit of a meagre bounty for a find of some particular severity.
You can argue about whether $4,500 is, or is not, reasonable. But as you may notice, it's not like he held the bug "hostage" threatening to sell it to the black market otherwise, or even hinting at perhaps doing that the next time.
He can be wanting to make the world a better place (or Facebook a safer site), and still be dissatisfied with the reward he got.
Even rationally: for instance, one could reason that even though they could sell the bug for $20k on the black market, this could cause damage and harm to innocent people on a scale (and with a certainty, because they won't buy the bug to just sit on it) that isn't worth the extra money he could gain (and for people with a decent moral compass that threshold is passed easily without even having to consider the risks of dealing with the black market etc). Then, having come to this conclusion, one could say "Well, I did the right thing, and I am disappointed that Facebook isn't offering more incentive for others to do the same".
Also, the incentive is not just for convincing people to give the bug to the good guys instead of the bad guys. It's clear from the article that he spent a deliberate and non-trivial amount of effort on finding this bug, he didn't just stumble across it. This Facebook bug was found because they offered a mystery prize, prompting him to search for a bug and dig deep, then he got disappointed that the prize was less than he anticipated. It's not so much a question of "will he sell it to the bad guys next", rather it's "will he bother going through that much trouble again to help fix a flaw in Facebook, knowing what the payoff will be?"
That said, my question: is $4,500 really too little? How much hours did this cost him (times a price appropriate to his specialist knowledge, of course), so how much should it have been, for doing the right thing? 2x, 3x this? I don't think straight up matching it up to black market prices really makes a lot of sense, there's too many external factors that make it a very different deal.