Earlier quoted context omitted.
Even if they did "pin it on him," why should he care, exactly? Seems to me like he had no obligations here, contractual, ethical, or otherwise.
Well because if he was taken to court that costs time and money, even if ultimately the lawsuit fails, no?
- if he can prove the threat, the company will be in an even worse position
- given the rather lacking precedent (as pointed out by the sister comment), there's a good chance the suit would be thrown out pre-court and therefore be rather cheap
- given that the OP seems to be a valued engineer, it is reasonable to assume that he has the f-you-money to fight this out
- even if won, the drawn out battle would take far too long to fix the software in time and, given that they then lost a 10 million contract, they lack the motivation (and maybe money) to continue
- lastly, even if they won after loosing the contract, they'd probably not be able to extract much in damage payments
So it's a rather empty threat.