I’m pretty sure he didn’t make that an ex cathedra statement. And since it’s not Christian theology, he has no ability to make such a statement.
anyone who supports or defends a religion that denies that Jesus is the Christ, the Messiah, is from anti-Christ
1Jn 4:1 Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world.
1Jn 4:2 Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God:
1Jn 4:3 And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that [spirit] of antichrist, whereof ye have heard that it should come; and even now already is it in the world.
1Jn 2:18 Little children, it is the last time: and as ye have heard that antichrist shall come, even now are there many antichrists; whereby we know that it is the last time.
1Jn 2:19 They went out from us, but they were not of us; for if they had been of us, they would [no doubt] have continued with us: but [they went out], that they might be made manifest that they were not all of us.
1Jn 2:20 But ye have an unction from the Holy One, and ye know all things.
1Jn 2:21 I have not written unto you because ye know not the truth, but because ye know it, and that no lie is of the truth.
1Jn 2:22 Who is a liar but he that denieth that Jesus is the Christ? He is antichrist, that denieth the Father and the Son.
1Jn 2:23 Whosoever denieth the Son, the same hath not the Father: [(but)] [he that acknowledgeth] [the Son] [hath] [the Father] [also].
I don’t know if it was in a cathedral or not when he said it, but it was a pope, talking about a matter of faith and religion. And he was very fallible after all.