It tells me, according to the %'s of correct answers, the average less than 50% of Americans know what they are talking about concerning religion, both their own and others.
It tells you nothing of the sort. I would predict that almost every single Hindu got every question about Hinduism right, almost every Muslim got every question about Islam right, almost every Jew got every question about Judaism right, and so on, because there was only one or two of each. This means the majority of all these could well know their own religion inside out, for all you know. There were far more questions on Christianity, so Christians fared less well, but they still, on average, got slightly over half the questions about their own religion right. Mormons and white Evangelicals did better on the Christian questions than did atheists.