The argument is that the ecumenical, papal affirmed body of cardinals deciding who is the valid pope is not an infallible decision, even though if it was one defining true doctrine it would be.
The College of Cardinals does not have the charism of infallibility. The Pope acting on his own can (the "Extraordinary Papal magisterium"), and an Ecumenical Council can. (An Ecumenical Council requires all bishops in the world to at least be invited; a group of cardinals only would not qualify.)
I'm not sure what "an ecumenical, papal affirmed body of cardinals deciding who is the valid Pope is not an infallible decision" even means.
Are you referring to a Papal election? That is "infallible" in the sense that, once a validly elected Pope has validly accepted election, the College of Cardinals can't simply reverse their act "because we want to".
Perhaps you have a fundamental misunderstanding of "infallibility". An "infallible act" is not guaranteed to be perfect or ideal, merely free from doctrinal error and therefore irreversible (although perhaps subject to better explanation and broader understanding -- as I said, it's not necessarily perfect).
If you are referring to a Pope being deposed because either (a) it is determined that he was not validly elected; or (b) it is determined that he is a contumacious public heretic and has thereby relinquished his office, that would require an Ecumenical Council, not merely an agreement by a group of Cardinals. I'm still not sure what "infallibility" would mean in that context. It's an administrative act, not a doctrinal one.
I hope that defense wasn't too "venomous".
Yes, although some RCs argue otherwise, I believe you are correct on this (as representing RC teaching), and my hasty remark was contrary with how I myself have described collegial infallibility, even though it seems that how many bishops are necessary for collegial infallibility is subject to debate. Sorry for so speaking off-the-cuff.
I'm not sure what "an ecumenical, papal affirmed body of cardinals deciding who is the valid Pope is not an infallible decision" even means
Correctly insofar as the above, it means an ecumenical body of cardinals deciding who is the valid Pope is, and confirmed by the pope (via acceptance), is not held to be an infallible exercise of the Sacred Magisterium.
Are you referring to a Papal election? That is "infallible" in the sense that, once a validly elected Pope has validly accepted election, the College of Cardinals can't simply reverse their act "because we want to".
Here is some of what Robert Siscoe at https://onepeterfive.com/dogmatic-fact-francis-pope/ argues,
The renowned Dominican theologian, John of St. Thomas, wrote what is likely the most thorough treatise of the peaceful and universal acceptance of a pope that has ever been penned, explaining each aspect of the doctrine with Thomistic precision. He compares the election of a pope by the cardinals to a doctrine defined by a council. He then explains that just as the infallibility of a conciliar decree is dependent upon its acceptance by the Roman pontiff, so too the infallible certitude that the legitimacy of the man elected by a conclave is dependent upon his acceptance by the Church. In both cases, it is the acceptance that ultimately provides the infallible certitude, and which renders the proposition de fide. Because of this, John of St. Thomas goes on to say:
Wherefore, if the Cardinals elect him in a questionable manner, the Church can correct their election, as the Council of Constance determined in its 41st session. Hence, the proposition [i.e., that the one elected is the true pope] is rendered de fide, as already has been explained, by the acceptance of the Church, and that alone, even before the Pope himself defines anything. For it is not [just] any acceptance on the part of the Church, but the acceptance of the Church in a matter pertaining to the faith, since the Pope is accepted as a determinate rule of faith.”[2]
The Legitimacy of a Pope is a Dogmatic Fact. As soon as the entire Church accepts the man as pope, his legitimacy becomes a dogmatic fact, which is a secondary object of infallibility.
But as per the OP article, that would require the consent of the pope, due to the power Rome has ascribed to that office.
I hope that defense wasn't too "venomous".
Not at all. Precision is important.