I take it that Steven's argument is as follows:
1. Christians critiquing stories from the Book of Mormon or the Koran use the principle that if a passage in a later document bears a great deal of similarity to an earlier story, the later story was plagiarized and the later story is fictional.
2. But the New Testament contains stories that are similarly similar to earlier accounts in the Old Testament.
3. Therefore, those New Testament stories were plagiarized and are fictional.
I am not sure about the principle. It is a question of the degree and relevance of the similarities. History does repeat itself. Sometimes when I am listerning to the news about Iraq I could swear that I've heard this all before. And I have, in news reports about the Vietnam was. Does that mean the news reports in the 2006 were plagiarized from those in 1969? As Yogi Berra said, it's deja vu all over again.