No it isn't. The contract was never signed. Terms sheets generally are not binding agreements.
Note that the agreement wasn't even ready for Alford to sign until after his resignation. My speculation is that the lawyers scrambled to throw it together quickly after being notified of his resignation.
If the terms sheet was intended to be a binding document, the actual agreement which was prepared after his resignation would be unneccessary.
Without having signed the actual new agreement, his employment would be governed by the existing agreement and addendum, which clearly does not have a liquidated damages provision for resignation, or the demand letter would not be citing the terms sheet.
However, if I were Alford, I'd be real concerned if the jurisdiction for dispute resolution is New Mexico. Put him up on a jury trial and they will make him pay through the nose--possibly treble damages for fraud, if NM has a statute to such effect.
That is why I think Alford will end up paying something. If it were not for the home court advantage, UNM would not have much of a leg to stand on.