I wish it were a joke. But respected academic and other surveys back up that and other signs of religious illiteracy. I've covered some as they've appeared. Here are a few lines I wrote a while back that have the source for “Ten percent of Americans believed that Joan of Arc was Noah’s wife":

"Stephen Prothero in Religious Literacy: What Every American Needs to Know — and Doesn’t (HarperOne, $24.95). He’s citing research published in George Barna’s The Index of Leading Spiritual Indicators (Dallas: Word Publishing, 1996)." At the time Prothero chaired the religion department at Boston University. I'd imagine that the number would be much worse today because it's much harder to teach even basic religious literacy in schools now that it's become so controversial.