Incredible. I would love to view tapes of his shows with him present so I could ask him, "How did you know that? When did you learn it?". Too many times I find myself thinking that he knows too many things that I have never even heard of.
From the linked article, Jennings was a member of the BYU College Bowl Team and is active in editing literature quiz questions. He described on a recent show using flash cards to learn mixed-drink recipes. He evidently gets a lot of practice learning facts.
I don't think he's practicing... he already seems to know the facts... and the questions!