Father, for my own clarification since I have absolutely no legal training…
Am I correct in believing that the word “derogate”, as used here, means to make (a provision of) a law null?
Close, but not exactly.
It means that it makes an exception to the law, but that the law itself still exists.
In more practical terms: what it means is that the rubrics and norms for the 1969 Rite of Baptism (i.e. the current Rite in use today, even though there were some minor changes since then) still retain the force of law. Pope Benedict did not abrogate the 1969 Rite. It remains in force, and it remains the Ordinary Form.
However, within the limited context of celebrations done according to the Extraordinary Form, the old rubrics and norms are the ones that apply, even though they obviously conflict with those of the 1969 Rite.
Put simply, it means that he was saying “if you follow the rubrics of 1962 when celebrating Mass in the Extraordinary Form, you are not violating any laws of the Church, because I specifically give you permission to do this, in fact, I obligate you to do it this way.”
Take an example from the Mass. The 3rd Typical edition of the Roman Missal (the current one, in English) says that there is to be only one opening Collect. No more than one. That’s still the liturgical law. However, if the Mass is being celebrated according to the Extraordinary Form, the older rubric that allowed for more than one Collect is the law that applies—even though those 2 sets of rubrics conflict with each other.
Make sense?