From Wikipedia:
Ex cathedra: In connection with papal infallibility, the Latin phrase ex cathedra (literally, “from the chair”) has been defined as meaning “when, in the exercise of his office as shepherd and teacher of all Christians, in virtue of his supreme apostolic authority, (the Bishop of Rome) defines a doctrine concerning faith or morals to be held by the whole Church.
Speaking on matters of doctrine or morals applying to the Church is IMPLIED in the term.
Wikipedia isn’t the catechism. Ex Cathedra means nothing more than from the office. Even if I accede to your definition (I don’t) under your usage, he can never speak Ex Cathedra concerning the doctrine or morals of another religion.