Vatican I defined Ex Cathedra statements are those which meet the following conditions, The Roman Pontiff speaks on a matter of faith or morals to the Universal Church. St. Thomas Aquinas was never a Pope, so he never could meet these conditions.
Therefore, faithfully adhering to the tradition received from the
beginning of the Christian faith, to the glory of God our savior, for
the exaltation of the Catholic religion and for the salvation of the
Christian people, with the approval of the Sacred Council, we teach
and define as a divinely revealed dogma that when the Roman Pontiff
speaks EX CATHEDRA, that is, when, in the exercise of his office as
shepherd and teacher of all Christians, in virtue of his supreme
apostolic authority, he defines a doctrine concerning faith or morals
to be held by the whole Church, he possesses, by the divine assistance
promised to him in blessed Peter, that infallibility which the divine
Redeemer willed his Church to enjoy in defining doctrine concerning
faith or morals. Therefore, such definitions of the Roman Pontiff are
of themselves, and not by the consent of the Church, irreformable.
So then, should anyone, which God forbid, have the temerity to reject
this definition of ours: let him be anathema.
Text from http://www.ewtn.com/library/councils/v1.htm