Section § II’s Parts and Ministers has nine numbered sections. We get legislation on the structure of this liturgical ritual. There are three required parts, and one optional:
6. The prayer of the faithful consists of several parts:
- the announcement or statement of the intentions;
- the responses of the assembly;
- the concluding formularies.
Another part that may be included is an introductory commentary.
The “introductory commentary” has become rather standard practice, at least in North America. Notice the wording here: it “may” be included, but it is not required.
Catholic Sensibility 
In my experience, the introduction is not “commentary” in the usual sense, but a brief ritual sentence leading the proceedings, sometimes as short as “Let us pray”.