As many readers may remember, the NLM always supported this view, which seemed the only canonically possible (see articles here, and here).
The Archdiocese of Milan, on the other hand, and in particular the Vicar for the Evangelization and the Discipline of the Sacraments, Msgr. Manganini, obstinately opposed this thesis, and tried to force everyone willing to celebrate the extraordinary form of the Ambrosian Rite to admit that the Motu Proprio didn't apply to it.
In this regard, it is important to point out that Msgr. Manganini's argument, based on the subtitle added to the MP "De usu extraordinario antiquae formae Ritus Romani", has now been openly belied by the Commission's new response.
The case considered by the PCED was important and revealing, because it highlighted a blatant injustice against Ambrosian Rite faithful.
In fact, an American priest living and incardinated in the bi-ritual Diocese of Lugano, and serving for a church whose faithful belong both to the Roman and to the Ambrosian part of the Diocese, pointed out that, whereas he, under MP "Summorum Pontificum" provisions, was undoubtedly free to celebrate the extraordinary form of the Roman Rite for his young Roman faithful, he wasn't sure whether he could consider himself also free to do so also for the Ambrosian Rite faithful.
He also pointed out that he considered pastorally and morally unjust that the faithful of the Ambrosian Rite be deprived of the fruits of the Holy Father's generosity.
This case was presented to the President of the Pontifical Commission "Ecclesia Dei", His Eminence Darìo Card Castrillón-Hoyos, during an audience last January.
And the answer of the commission, signed by the Vice-president of the Commission, Msgr. Camille Perl, is very clear.
And here's the translation of the text by Fr. Zuhlsdorf, slightly modified:
Your letter of January 7, 2009 has had our attention but is still waiting for a reply. You express a desire to “receive the comfort of having the approval of the possibility that also your Ambrosian students (that is, of the Collegio Papio at Ascona) who ask for it can enjoy the benefits guaranteed by the Holy Father” in the Motu proprio “Summorum Pontificum”.
While it is true that the Motu proprio of the Holy Father does not expressly cite the Ambrosian rite, it doesn’t exclude the other Latin rites; if the will of the Holy Father asserts for the Roman rite, considered superior in dignity, consequently much more for the other Latin rites, including the Ambrosian rite.
Wishing the blessings of the Lord on your pedagogical work, I greet you fraternally.
A Solemn High Mass according to the extraordinary form of the Ambrosian Rite will be celebrated next week in the Archdiocese in thanksgiving for the restoration of the true rights of the Ambrosian Rite.
Quid retribuam Domino pro omnibus quae retribuit mihi?
Calicem salutaris accipiam, et nomen Domini invocabo
Laudans invocabo Dominum, et ab inimicis meis salvus ero.