Yes, of course the Trinity, the Incarnation are foundational truths of the Faith and necessary for salvation, and he who does not believe is condemned already. But the real issue is different.
The point is that the worship they offer is in vain, for as the Scripture says, it is following the commandments of men, and it is not worship offered in spirit and in truth, as Catholic Christian worship is. For the same reason, Jєωιѕн worship too awails nothing.
Muslims recognize and attempt to worship the God of Abraham. It is impossible for us to say that the God of Abraham is not the true God. Insofar as someone says, The God of Abraham is the true God whom Christians worship, he is right. However, they are ignorant of the fact that the God of Abraham is the Holy Trinity.