I think that the Natural Law is best summarized by Christ’s commandments to love God and to love one another. It’s a little more than “do good, avoid evil.” I think it is also a little more than do unto others. The command to love God is part of the natural law, I think, but it is perhaps better summarized as ‘recognize that there is something beyond yourself, that is greater than you are’. So some cultures have responded imperfectly to this law by worshipping false gods or by worshiping ideals.
But your base point I agree with strongly. I think that many people toss things into “Natural Law” that are more accurately described as "derived from Natural Law principles. There is a big difference. The more I look at it, the more I am convinced that the Natural Law only encompasses a very small set of very broad rules, i.e. love God, and love one another.
We also have revelation, which adds more rules and substance to the Natural Law, but we must remember that even these rules must be interpreted in light of the first two. This is where the Decalogue fits, along with a very few other things.
Everything else is the result of reasoning from those principles. Like all products of human reason, these laws can be imperfect or flat-out wrong. Many very bad laws that we now reject were said to be “Natural Law” at one time.
I think that those are all consistent with what I said above. The Card. Ratzinger document you quote from quotes in turn from Irenaeus for the proposition that the Decalogue is a rationale and reasoned ethical code derived from the Natural Law and given to help man live in accord with Natural Law. As a “preparation for friendship with God and justice towards our neighbour.” The morality that is drawn from the Decalogue is similarly a rationale construct, now one more setp away from Natural Law.
As we develop more detailed rules, we naturally take more steps from the source. At some point, we need to check our backtrail and make sure we have not strayed from first principles.
Some people refer to all rules along these steps informally as “Natural Law,” but they are merely derived from, or reasoned from Natural Law.