Your point about not offering unnecessary information is, unfortunately, good advice.
In my situation it felt as if the officer was going to every possible length to find a reason to reject my application.
I was explicitly asked why I was no longer with my former employer, what my last day of work was, and what date I had previously left the US to return to Canada. Being untruthful about any of these easily-verifiable points would have been much worse in the long run (since this officer definitely seemed intent on running down every possibility). Honestly I am lucky that I had a proof of my severance agreement, which showed it was not a voluntary departure.
Did you tell the agent you didn't leave after you were laid off? Or did you just ask for a new TN once you had a new offer?
It seemed to me like being too truthful could hurt your chances more than to just ask for a new visa and let them process the TN as needed.
Some employers wouldn't even report your termination date until some time in the future to give you time to find a new job.