"The Church in this world is the sacrament of salvation, the sign and the instrument of the communion of God and men" (CCC 780).
The phrase "no salvation outside the Church" means "no salvation without the Church." That is, all salvific grace comes through Christ's redemptive death on the cross, which is mediated through the Church, Christ's Body.
For example, in the sacrament of Holy Eucharist, Christ's death on the Cross is re-presented when Christ, acting through the person of the priest, offers up His Body to the Father in an unbloody sacrifice, under the appearances of bread and wine.
Normally, sanctifying grace is first received in Baptism where we enter into the mystery of Christ's death and resurrection.
It appears what Catholic Answers is saying the Catechism modifys the Infallible Teaching of the Pope. No?