Derbyshire does go too far, but I also wondered why Ben got involved with this.
You can see something similar with Ann Coulter. She goes to the extreme, even when she doesn't have to.
Because the producers, Logan Craft, Walt Ruloff, and John Sullivan needed someone who wasn't overtly Christian:
TEXAN: How did Ben Stein come to be involved in the film?-- Q&A: ‘Expelled’ producer Logan Craft
CRAFT: Well, John had a real insight, we believe, into the necessity to have a person, first of all, who wasn’t overtly Christian or overtly religious and also someone who had a comic element to their personality or their repertoire, but also an intellectual. Well, that kind of limits the field. There aren’t that many of those folks out there.
Once Ben became acquainted with what we were doing, he got excited because he began to see a connection between our exploration and sanctity of life issues. He’s a very, very strong pro-life advocate. He has a high view of human dignity and human sanctity. And he saw a connection between what we were exploring, and sanctity of life issues and the historical elements of the eugenics movement, and especially as a Jewish person, the eugenics movement as it morphed into the Nazi racial cleansing laws. ...