The category error is the implication that some of the faithful are not sinners. Scripture is clear that all have sinned - all are sinners. Now, perhaps someone will come here and tell us that the pope is using special papal jargon and that "sinners" here means only those with especially serious sin.
If one's goal were simply to try to find an orthodox interpretation of the pope's words, one might even point to a passage of the speech a bit further down where it spoke of the "conversion of sinners" as supportive a special sense of the term.
But that would be a stretch. It's more probable that the pope was committing the same category error we see in the "Catechism of the Catholic Church":
2634 Intercession is a prayer of petition which leads us to pray as Jesus did. He is the one intercessor with the Father on behalf of all men, especially sinners. He is "able for all time to save those who draw near to God through him, since he always lives to make intercession for them." The Holy Spirit "himself intercedes for us . . . and intercedes for the saints according to the will of God."- CCC 2634
The catechism is wrong. He does not intercede for all men (at least not "all" in the sense of "each and every"), but only for the elect. He does not intercede for any but those who need intercession, namely sinners. He is the only intercessor for all men, but he is not the intercessor for each man. So likewise, all men are sinners, and thus the "especially sinners" would not (but for the faulty presuppositions implicit) provide any further limitation on his intercession.
The bottom line? No matter what the pope or the catechism say, Scripture tells us:
Romans 3:10 As it is written, There is none righteous, no, not one: