The Church has previously defined that Mary is immaculate.
Pope St. Martin I, Lateran Council, 649 A.D., Can. 3- “If anyone does not properly and truly confess in accord with the holy Fathers, that the holy Mother of God and ever Virgin and immaculate Mary in the earliest of the ages conceived of the Holy Spirit without seed, namely, God the Word Himself specifically and truly, who was born of God the Father before all ages, and that she incorruptibly bore [Him], her virginity remaining indestructible even after His birth, let him be condemned.” (Denzinger 256)
Not just because the teaching was found, it means that it was a dogma at the time. I am pretty sure every Catholic agrees that the dogma of the Immaculate Conception was defined as such in the year of 1854 in Pope Pius IX's Bull
Ineffabilis:
We declare, pronounce and define that the doctrine which asserts that the Blessed Virgin Mary, from the first moment of her conception, by a singular grace and privilege of almighty God, and in view of the merits of Jesus Christ, Saviour of the human race, was preserved free from every stain of original sin is a doctrine revealed by God and, for this reason, must be firmly and constantly believed by all the faithful.
Anyway, even if St. Thomas did err on this point, I really don't think he was a heretic, less a
formal heretic.
Even if you do not agree with a saint; or do not "feel" veneration" towards him, accusing a canonized saint of the caliber of St. Thomas of nothing less than heresy is way too much. It really gives a bad name to Traditional Catholics.