Yeah, if you read the letter informing workers of the layoff, it's clear that their severance depends on continued good performance until a separation date. Good performance includes training the replacement.
The one time something vaguely like this happened to me, granted my boss made it very personal although in truth he'd lost his ability to sell, I would have been much, much better off telling him to FO and leaving on the spot, instead it triggered anxiety that eventually disabled me, which I later learned I had a genetic predisposition towards.
Money isn't the end all and be all, after all.
"Please your honor, it's such an easy temptation to fall into, when the articles are so infrequently more useful than the comments section."
More likely, if they'd left immediately, many of them could have gotten the equivalent of 4 months wages at their new job, since hopping is often the best way to get a raise.
As long as you answer all of their questions you would certainly be meeting the good performance stipulation. There would be no way for them to know whether you are providing the whole story. Why offer up information that is not specifically requested? After all, the hardest part of engineering is knowing which questions to ask in the first place.