The proposition is equivalent to "The Son of Mary was not God" which is heretical. The denial is direct and unmistakable.
THANK YOU!
"Muslims together with us worship the one merciful God, mankind's judge on the last day"
Is equivalent to "Jesus Christ is not the one merciful God, mankind's judge"
But you deny this because it first involves the knowledge first that Christ is the judge (which all must know in order to be saved) and second that Muslims reject His Divinity.
You assert that the denial is not direct, even though it is the logical conclusion, requiring one step of reasoning, and only one. This step of reasoning does not make the proposition any less heretical, because the conclusion is in contradiction to a dogma.
So the proposition needs to be censured in some way, and the only way is heresy. The next step down from this is proximate to heresy, that is the denial of a truth that is proximate to the Faith, but the proposition from
Lumen Gentium simply does not qualify for this censure, since the nature of Christ as Judge is already proposed as Divinely revealed.