Excellent point. Indeed Islam is a heresy.
To me, it sounds as if it is not:
“Heresy is the obstinate post-baptismal denial of some truth which must be believed with divine and Catholic faith, or it is likewise an obstinate doubt concerning the same; apostasy is the total repudiation of the Christian faith; schism is the refusal of submission to the Roman Pontiff or of communion with the members of the Church subject to him” (CCC 2089).”
Doesn’t the above exclude Islam? Or am I wrong?
Not according to your article. According to your article from the Catholic Education site that carriers the NIHIL OBSTAT, heresy requires that the proposed heretical claim be held and professed by one who is Baptised. Since the Islamics don't practice Baptism, their claims are simply another religion and not a heresy.
From the article: "A person must be baptized to commit heresy. This means that movements that have split off from or been influenced by Christianity, but that do not practice baptism (or do not practice valid baptism), are not heresies, but separate religions. Examples include Muslims, who do not practice baptism, and Jehovah's Witnesses, who do not practice valid baptism."