Well, if the missal does not mention it, it cannot be added, right? This is how I understand it. The missal must be followed scrupulously. Nothing can be added or removed.
Strictly speaking, the communion of the faithful is an interruption of the Mass, not part of it, and the second
Confiteor is part of that interruption. To treat it as an illicit "addition" would mean that none of the faithful could ever receive communion at Mass. (Indeed, in times past, communion was often administered outside of Mass.)
Traditionally, the sermon has been seen the same way, as an interruption, though the 1983 CIC refers to the sermon as an integral part of the Mass (presumably they have in mind the Novus Ordo).