You would do well to read further in the linked document:
But first it must be clearly understood whom we have to oppose and contend against, and what are their tactics and their arms. In earlier times the contest was chiefly with those who, relying on private judgment and repudiating the divine traditions and teaching office of the Church, held the Scriptures to be the one source of revelation and the final appeal in matters of Faith. Now, we have to meet the Rationalists, true children and inheritors of the older heretics, who, trusting in their turn to their own way of thinking, have rejected even the scraps and remnants of Christian belief which had been handed down to them. They deny that there is any such thing as revelation or inspiration, or Holy Scripture at all; they see, instead, only the forgeries and the falsehoods of men; they set down the Scripture narratives as stupid fables and lying stories: the prophecies and the oracles of God are to them either predictions made up after the event or forecasts formed by the light of nature; the miracles and the wonders of God's power are not what they are said to be, but the startling effects of natural law, or else mere tricks and myths; and the Apostolic Gospels and writings are not the work of the Apostles at all. These detestable errors, whereby they think they destroy the truth of the divine Books, are obtruded on the world as the peremptory pronouncements of a certain newly-invented "free science;" a science, however, which is so far from final that they are perpetually modifying and supplementing it.
I did, but I don't hold Pope Leo XIII to be infallible.The point I was refuting was the myth that most or all of the Bible has already been infallibly been interpreted by the Magisterium. It's clear from the Catholic church's own prounouncements that this is not the case.