Not every statement from a pope is ex cathedra. In fact, most popes NEVER make such a definition. Do you understand this? Honorius' letter was not intended by him to be a new teaching since he recommended the issue be kept silent. Therefore it most certainly was not an ex cathedra definition.
You say Honorius must have made such an ex cathedra definition because the Third General Council of Constantinople said so? Not really. The council was in the midst of fighting Monothelitic heretics that had found Honorius' letter and used it as a pretext to revolt. Heresies at that time were used as the cover for political revolts (and still are). The harsh condemnation was certainly a polemic to combat the heretics and probably exaggerated Honorius' offense.
Even so, the council never accuses Honorius of defining the heresy. Honorius and his friend had exchanged letters musing over some very confusing topics about which both writers were obviously confused. Honorius never reached any firm conclusions or intended that any new doctrine be taught. Here is a discussion of the issues in the letter:
Please don't put words in my mouth. I did not say Honorius "...must have made such an ex cathreda definition...". I did say he was convicted of Heresy by an Infallible Ecumenical Council. Further, I said the "ex cathreda" definition was not yet invented and was conveniently applied as required on a retroactive basis.
IOW, your Honorius argument is based on a fraudulent argument.
You and I are wasting our time on this subject and I am perfectly willing to drop it. Good luck with the Kool-Ade.