To your first point: There's being skeptical and then there's calling someone a liar without actually calling them a liar because you don't have any justification for doing so. This is far from the first time I've seen this on HN and it's really not okay. There's no point in speculating about the veracity of this person's statements until there's a reason to.
To the second and third: They only require that a researcher "...make a good faith effort to avoid privacy violations..." and I'd say he met that. You can argue that the entire endeavor wasn't in good faith but he certainly made a significant and conscious effort to avoid private data.
I think his biggest lapse in judgement was that he brought security operations issues to light in a bug bounty program run by the people that would be most embarrassed by them. Application security bugs are created by the engineering team and the CSO's application security team fixes them (or advises or whatever). Security operations issues are entirely the responsibility of the CSO's department.
Facebook (as an organization) should be thanking him. While he didn't expose application security bugs he exposed significant operational issues and blind spots. Keys with far too much access, lack of log inspection, lack of security around what IP addresses a key can be used from, etc. Operational issues and lapses in operational security are what got Twitter in hot water with the FTC in 2010. It's not as easy to play cowboy with operations as it used to be.
The CSO hasn't been around for long but by all accounts he poured a lot of effort into hiring an application security team. Perhaps that's his specialty but even one experienced technical manager hired for security operations could have caught these basic issues. They probably wouldn't have addressed the lack of least privilege in that time frame but they could have easily spun up logging to catch some rando on an unknown IP address using their keys.
But like I said, he hasn't been there for long so I don't blame him for the failure. What I do blame him for is calling up the employer to threaten them as leverage to shut up the researcher. I blame him for posting a thinly veiled justification for doing so. He could have addressed this openly, talked to the guy directly and went to the other C-level execs with it as a justification for getting everyone on board with fixing it but he tried to keep it contained to his department.
I understand how he must feel being the new guy who's responsible for the outcome but not for creating it. I know he'll get questions that he might not be able to answer since they probably aren't logging bucket access. Questions like, "Who else got a copy of these keys and what did they access?" Saying "I don't know and we may never know" in response to that, even if you weren't in charge more than three months ago, is rough.