Simpler: he's saying that there are people who believe in something, but they don't claim to know it.
For example. I brew some coffee at 14:00. Now it's 18:00. I believe that my coffee is still warm, but I don't know it - because I have no data to back up that knowledge. I can however generate said knowledge by grabbing a cup of coffee. (I just did it. It's warm.)
What the agnostic theists do is like that. With a key difference: they cannot generate said knowledge, and they know it. They cannot grab that cup of coffee.