Here is the article from Document.no about the Norwegian gunman. Most of the site is in Norwegian, but this article is in very good English.
It is actually the author who gets the Cultural Marxism aspect wrong.
His main enemy is not muslims, but multiculturalists and what he calls cultural marxists. This is a term that some on the political right uses. It is vague and can be used about almost anything. Because it is so imprecise this editor raised an inner red flag when Behring Breivik used it about the Frankfurter School. This was an important philosophical school in Germany, after the war, led by Adorno and Horkheimer. That Behring Breivik classified them as cultural marxists indicated that he did not really know cultural history. But that does not make him an extremist.
Actually, the term is not really “vague,” and it is entirely appropriate to use Cultural Marxism to describe what came out of the Frankfurt School. The Frankfurt School essentially originated and popularized it.