As an example of an erroneous statement, he quotes Lumen Gentium 16 which claims that salvation includes "those who acknowledge the Creator” like the Muslims who along with us “adore the one and merciful God.”
Viganò explains that the god of Mohammed is not one and triune and that Islam condemns Christ's Incarnation. From this, he concludes that certain Vatican II statements are heterodox and cannot be a part of the Magisterium which cannot contain errors.
Islam is a mixture of error and truth, the latter being where it’s beliefs about God are consistent with the Catholic Church. God may allow infidels to see some aspects of the truth and thereby be led ultimately to the true faith, as part of God’s plan of salvation. That is why we pray for those who do not believe in Christ separately from those who do not believe in God on Good Friday.