While I do agree that Tripp does not go as far as the others - he may very well fall short of effectively claiming means of grace level for spanking, although I'm not so sure it is something to boast about - he is nowhere near orthodox.
Why? Because of this: Tripp claims that "God commands spanking":
The use of the rod is an act of faith. God has mandated its use (109).
Spanking is not "one tool among many" to Tripp, but THE tool. "Spanking" is now synonymous with "discipline". As far as Tripp is concerned, if you don't spank, then by definition you aren't disciplining:
If you fail to spank, you fail to take God's Word seriously. You are saying you do not believe what the Bible teaches about the import of these issues. You are saying that you do not love your child enough to do the painful things that God has called you to (149).
And if you aren't disciplining, then you are disobeying God, and your children are effectively lost:
The rod is a rescue mission. The child who needs a spanking has become distanced from his parents through disobedience. The spanking is designed to rescue the child from continuing in his foolishness. If he continues, his doom is certain. Thus, the parent, driven by love for the child, must use the rod (110, emphasis mine).
Spanking cannot guarantee your children's salvation, Tripp says, but NOT spanking will pretty much doom them. How can that be, unless spanking has some sort of power that no other method of discipline has? (Of course, according to Tripp, there isn't any other method of discipline: Spanking is the only way mandated by God.) But never fear, Tripp has the obvious answer: he says spanking DOES have power - the power to change the heart:
[S]panking enables you to deal with issues of the heart....The heart is the battleground. The spanking comes only because it is God's method of driving foolishness far from your child's heart (153).
The child is a sinner. There are things within the heart of the sweetest little baby that, allowed to blossom and grow to fruition, will bring about eventual destruction. The rod functions in this context. It is addressed to needs within the child. These needs cannot be met by mere talk....[F]oolishness is bound up within his heart. Allowed to take root and grow for 14 or 15 years, it will produce a rebellious teenager who will not allow anyone to rule him. The spanking process drives foolishness from the heart of a child. Confrontation with the immediate and undeniably tatile sensation of a spanking renders an implacable child sweet(105-106, emphasis mine).
The rod returns the child to the place of blessing. Left to himself, he would continue to live a lust-driven life. He would continue to seek comfort in being a slave to his desires and fears. The rod of correction returns him to the place of submission to parents in which God has promised blessing (115).
There you have it, folks. Spanking is "God's method of driving foolishness far from your child's heart". "It is addressed to needs within the child." "These needs cannot be met by mere talk." "The child is a sinner...Left to himself, he would continue to live a lust-driven life." "Thus, the parent...must use the rod." "The rod returns the child to the place of blessing."
So, Tripp may not say spanking saves, but he DOES say that it must be used, and not using it has catastrophic consequences. That is completely unorthodox, and is more than bad enough to warrant jettisoning his advice entirely. This is a good example of how ONE wrong doctrine can so thoroughly poison an otherwise correct book.