Nice try, but the Immaculate Conception was defined ex cathedra.
Now please address the issue instead of dodging it. What was it, if not the defining of the dogma ex cathedra(i.e by the Extraordinary Magisterium), that made St. Thomas not a heretic but someone who denies the same truth today a heretic?
The dogma was defined, not invented. The Immaculate Conception of Our Lady always was one of the doctrines of the Church, it was never some new idea, some new concoction or pious innovation that the pope in union with all the bishops of the world decided to make a dogma. But that is how you and Cantarella and Lad preach the whole process works. Ridiculous!
Yes, all the bishops and cardinals petitioned the pope that it was finally time to actually solemnly define it, but it was already something the Church always and everywhere taught since the time of the Apostles. This is what it says in Ineffabilis Deus.
This doctrine was always taught and believed because this doctrine was and will always remain in the ordinary and universal magisterium - were it otherwise, it could not have been solemnly defined. Do you understand this?
This means that the certainty we have of the Immaculate Conception of Our Blessed Mother, whether defined ex cathedra or not, was, is and will forever be, among those teachings included in
"all that has been handed down as divinely revealed by the ordinary teaching authority of the entire Church spread over the whole world," i.e. the ordinary and universal magisterium.
The Immaculate Conception was always one of those "points of doctrine" that Pope Pius IX taught in Tuas Libenter when he said we must submit ourselves to
"points of doctrine which, with common and constant consent, are held in the Church as truths and as theological conclusions so certain that opposing opinions, though they may not be dubbed heretical, nonetheless, merit some other form of theological censure."