I think it must be said that Archbishop Lefebvre did not add "it by his own authority because he wanted to."
In Church Law something takes on the force of law when it becomes a custom of many years (I won't get into precise numbers here because I don't know for sure what they are--but I don't think it is necessarily a precision issue). The Confetior prior to Communion began a few centuries ago as a pious practice in churches throughout the world. Rome later made it obligatory. Thus, it began through custom and not legal fiat.
The 1962 Missal removed the legal requirement. The SSPX was originally flexible on which Missal the priests used. Some used the 1962 Missal, others used an earlier Missal. In the United States, I understand that most Society chapels used the 1955 Missal. At some point, Archbishop Lefebvre standardized the Missal used in Society chapels to the 1962 Missal. Although the Confetior prior to Holy Communion is not in this Missal, the archbishop recognized that its use was of long established custom and, because many of the faithful wished its retention, he encouraged its inclusion in the Masses of Society priests. I don't think a priest who uses the 1962 Missal is necessarily wrong or right to include or exclude the Confetior prior to Holy Communion.
One additional comment: The Confetior said at the foot of the altar is not said solely for the servers. If there are no servers, the priest's Confetior is said on behalf of himself and the people. If there are servers, the Confetior is said on behalf of themselves and the people. The layman in the pew does not have to "say" the Confetior in order to receive the benefits of the forgiveness of venial sins.