I donāt believe in the infallibility of Pope, Church, ecumenical Councils, or of the episcopate in union with the Pope, simply because there is no reason to do so. Infallibility in teaching doctrine is in the eye of the beholder, which is why it impresses only those who believe in it.
I wouldnāt call it nonsense, so much as non-sense: if you analyse it, it doesnāt work. Itās a nice theory, & itās nice in theory: but itās meaningless in practice - it has never yet stopped a persecution or a massacre: these things can always be rationalised so as to become good & holy acts. So it is useless.

And that is just the beginning. IMHO, the definition of 1870 was a classic own goal, a blunder on a colossal scale, a disproof of the very idea that it defined. We have had to put up with the consequences ever since; thanks to Pius IX, we are trapped, so that we are doomed to venerate a colossal falsehood as though it were Godās truth. The moral results of this idea have been disastrous.