That all depends on what the meaning of "is", is!
Newt professes to be a Christian, and he expresses regret for his past adulteries. To say a person can’t change is to deny the power of God, plus it goes against what I have witnessed myself. I have personally seen people change, even very old sinners. It does happen. As far as I’m concerned, Newt’s past adulteries are between him and God. As you pointed out, he hasn’t denied his sins unlike some other politicians we know.