Yes, this is the usual SSPX response (propaganda, really) from the fallout of the expulsion of the Nine. There are, in fact, many deep problems with this reasoning that the Society has employed, but for brevity I will just cover two.
St. Thomas is indeed correct, but the real question is the minor: does the principle apply in this case? And does the SSPX even follow their own principle?
The SSPX themselves do not submit to lawful authority on the reforms of Holy Week, as I and many others before me have pointed out. John XXIII did not even submit to the authority of his predecessor in this case. Nor did many priests of the day who quietly refused to adopt the changes, and we commend them for it, for it was these same priests who several decades later would assist the SSPX in setting up her chapels in various places. The SSPX have maintained certain pre-reformed rubrics that were explicitly removed by Pius XII in the 1955 edition and further by John XXIII in the 1962 edition. In these particular rubrics, there is certainly no danger to the faith, yet the rubrics were maintained. The SSPX does not have the lawful authority to do this and, more so, violate St. Thomas's principle.
This point expands even more strongly to their use of the 1962 missal and 1960 office. The Society regularly disregards many rubrics of the reformed books. Most priests maintain certain pre-62 rubrics; many incorporate '65-'67 rubrics, up to and including senior priests of various districts. But again, contra St. Thomas and the authority of the Church, who alone oversees all rubrical matters, the SSPX, much less individual priests, does not have the authority to make such changes.
In fact, an impartial study of the case suggests strongly that the commitment to the reformed Holy Week as well as the 1962 missal is more an overreaction to the exaggerations of the Nine, whose actions threatened to undermine +Lefebvre's negotiations with JPII leading up to 1984. This was unacceptable. Most scholarly consensus on the Roman rite since the 1980s has shown again and again how the reforms are predicated on falsified "evidence", motivated often explicitly by strong ideological agenda, all in the direction of the Novus Ordo. The Pacellian Holy Week is filled with textbook examples of the very error of antiquarianism that the same pontiff condemned just years earlier.
The inability to move past this past "baggage" is the reason why I do not believe the reformed Holy Weeks will disappear anytime soon. And as the neo-SSPX has deepened their entanglement with Rome, they must double down on their apparent rejection of sedevacantism or anything that reeks of extremism in order to maintain favor with the Vatican.
The second problem, which I will not go too far into, is that while St. Thomas's axiom is necessary, it is not sufficient. And the reason it is not sufficient is easy to demonstrate. If we who have the benefit of hindsight were given a time machine to go back to 1948 to warn Pius XII of the chain of liturgical changes initiated by him that would eventually lead to the Novus Ordo, we would have no grounds to dissuade him by referring to St. Thomas's principle alone. In other words, the question of danger to the faith would have stopped none of the liturgical changes that led finally to the Novus Ordo, which was all done in the name of the same ideological principles and "scholarship". But we cannot accept the Novus Ordo. Therefore, at some point, an additional theological consideration has to be brought in to halt Pius XII. But what could we tell him that would possibly give him pause? We cannot rely on "feelings" of impending doom. We cannot say we don't "like" Mgr Bugnini or Fr Bouyer etc. We cannot merely say Jungmann and Fortescue were wrong (at this point, they weren't yet proven to be wrong). What theological principle could we posit? Fr Cekada attempted once to answer this, but I believe with mixed success. It requires greater minds than ours.
And that gets to the heart of the question, even until now unresolved, and may be for many more generations. The SSPX is not concerned with this question, unfortunately. But I have found the small experiences telling when meeting not a few individuals who grew up in the chapels and churches that used the pre-reformed Holy Week and who later attended Society chapels and were confused, even shocked, at the differences. For those who breathed in the atmosphere of the old Roman rite from their childhood, even the SSPX felt jarringly modern to these laity untrained in theology or liturgy. If you can say the Pater together one day of the year, why not every day? Nothing dangerous to the faith in that. It would be blasphemous to say otherwise, would it not? Ah, but now we have a dilemma that St. Thomas cannot resolve for us.
And I have met many who came from the Novus Ordo who told me that they felt the reformed Holy Week was a mostly Latinized version of what they already experienced in the Novus Ordo but felt the pre-reformed Holy Week very foreign.
These experiences, while anecdotal, are telling data. Is there danger to the faith? Well, that is a tricky analysis sometimes, as even our beloved Bp. Williamson will admit...