dmpoole... I love you man.
Gervais just remembers that stuff off the top of his head. It's only ever about anthropology or history, but he talks quite passionately about them and Merchant is the same too (he's more into art and philosophy). My wife and her entire family are the same, they just have ridiculously good memories and especially if it's regarding a passionate subject.
There's no way they had time to plan such a masterpiece. Why would they for a poxy poorly paid show on a tin box radio station like XFM. Gervais was working on the office and extras, how would he find the time and why would he bother when he's already established. What possible credit would he receive from putting in all that effort on a crappy radio station that only broadcasts to a tiny area (especially if he's already established).