I don't think anyone was claiming that being moral about the NDA does "weigh up" against working there in the first place. If he were trying to paint a glowing picture of his own morals, he wouldn't have written the article in the first place.
(Since he appears to have been there for something like 6 months before getting fired, I'll hazard a guess that his severance pay wasn't all that spectacular.)
10 K+ per month is not unusual, and depending on his contract they may have to 'buy you out' (for instance, after three months they might have signed a years contract with you including the first three months).
So the severance could be anywhere from $5K up to a very substantial amount of money. The fact that they're willing to pay another $16K just for signing an NDA suggests it was a pretty high salary.
He already said, kinda, what the salary was: with bonuses and whatnot, he expected to be getting $200k per year.
Given what he's possibly done to their reputation (I don't know; perhaps everyone already thought they were corrupt and dishonest anyway) that he wouldn't have been able to do if he'd signed the NDA, I don't see that an explanation in terms of salary is required: getting everyone to sign the NDA greatly reduces their exposure to this kind of bad publicity.
(Since he appears to have been there for something like 6 months before getting fired, I'll hazard a guess that his severance pay wasn't all that spectacular.)