IANAL, but as far as I know, a "nolo contendre" plea can be treated by the court as if it were a guilty plea which would open Libby up to the full punishment for the offenses charged.
It can be used both ways. If the defendant without urging from the prosecution pleads it, he could get zapped. Particuliarly if the case has gone to trial. I was selected on a jury in a fraud case where elders had been bilked by a bastard. The jury selection for the defendants was really bad. Somehow they had allowed two CPAs and 3 guys with MBA's to be enpaneled/selected. I was selected as Jury Foreman.
Suddenly one of the lawyers asked to approach the bench, after a few minutes of session with the Judge. The judge said he wanted to ask our education level. As noted 3 of us had MBAs and we had two CPAs with us. When the second CPA noted she was a CPA, the attorneys for the defendant and the DA approached the bench. In that case he pleaded "nolo contendre". The judge dismissed the jury and proceeded to sentence the defendant.
However, when the prosecution has a really weak case and wants to get out without a lot of egg on their face, they often will signal/tell the defense attorneys that a NC plea will mean the end of the case. All of the lawyers and defendant will then,often go the Judge's quarters and the NC plea is entered with no more proceedings to follow.