Wow. He says the exact opposite, multiple times. He says that not only do they offer -- though not through a visible liturgical act -- Christ the Victim through the hands of the priest, but they also offer themselves.
Now it is clear that the faithful offer the sacrifice by the hands of the priest from the fact that the minister at the altar, in offering a sacrifice in the name of all His members, represents Christ, the Head of the Mystical Body. Hence the whole Church can rightly be said to offer up the victim through Christ. But the conclusion that the people offer the sacrifice with the priest himself is not based on the fact that, being members of the Church no less than the priest himself, they perform a visible liturgical rite; for this is the privilege only of the minister who has been divinely appointed to this office: rather it is based on the fact that the people unite their hearts in praise, impetration, expiation and thanksgiving with prayers or intention of the priest, even of the High Priest himself, so that in the one and same offering of the victim and according to a visible sacerdotal rite, they may be presented to God the Father. It is obviously necessary that the external sacrificial rite should, of its very nature, signify the internal worship of the heart. Now the sacrifice of the New Law signifies that supreme worship by which the principal Offerer himself, who is Christ, and, in union with Him and through Him, all the members of the Mystical Body pay God the honor and reverence that are due to Him. -- Mediator Dei, section 93
In order that the oblation by which the faithful offer the divine Victim in this sacrifice to the heavenly Father may have its full effect, it is necessary that the people add something else, namely, the offering of themselves as a victim. -- MD, section 98
99. This offering in fact is not confined merely to the liturgical sacrifice. For the Prince of the Apostles wishes us, as living stones built upon Christ, the cornerstone, to be able as "a holy priesthood, to offer up spiritual sacrifices, acceptable to God by Jesus Christ."[89] St. Paul the Apostle addresses the following words of exhortation to Christians, without distinction of time, "I beseech you therefore, . . . that you present your bodies, a living sacrifice, holy, pleasing unto God, your reasonable service."[90] But at that time especially when the faithful take part in the liturgical service with such piety and recollection that it can truly be said of them: "whose faith and devotion is known to Thee,"[91] it is then, with the High Priest and through Him they offer themselves as a spiritual sacrifice, that each one's faith ought to become more ready to work through charity, his piety more real and fervent, and each one should consecrate himself to the furthering of the divine glory, desiring to become as like as possible to Christ in His most grievous sufferings. -- MD, section 99
Mere witnesses? Seriously?
I’m not sure it’s the exact opposite—at least as I read it. He seems to be saying that the Sacrifice is offered by the priest, alone, but the faithful have a role—but in their place—which is not at the same level as the priest. The people do this through uniting their “hearts in praise”, not by any physical act.
Vatican II places the “presider” on the same physical level as the faithful and is very likely moving toward having the extraordinary ministers physically offer the Sacrifice along with the priest. I’d be surprised if we don’t see that being permitted in the very near future. This is where they are going with it, and it has never been suggested in tradition.
Admittedly, the language is a bit murky on that issue, but he states that “this is a privilege “only for the minister.” We don’t know how involved Bugnini and Bea were when he wrote that encyclical, but I wouldn’t be surprised to learn that they were both very close by.