Call me a cynic, but the only possible reason he would not reveal "where certain bomb-making materials are still hidden, even to this day" is that there aren't any.
The purpose of a plea bargain is to close the books on a case with a notch in the prosecutor's belt.
The purpose of a plea bargain is NOT to find the truth.
My guess would be that Mr. Nichols signed whatever papers were put in front of him by the prosecutor, regardless of their veracity, just to get the death penalty off the table.