I got Kevin Smith laughing hysterically on stage once without even trying, I felt like I'd unlocked some life achievement, it was awesome. I can only imagine how great it would be to get a laugh out of a legend like Terry Jones!
He reasoned that since he's a professional, when he watches another comedian act, he pretty much knows what's going on under the hood. He can predict where a joke is going because there are tried and true formulas, he can see through their pacing, etc. But if a noob cracks one, even if it's a joke he's heard before, he knows there's probably none of the calculation that a pro puts into it. It was a genuine thing, which makes it all the more hilarious to him.
He was a generous laugh, and that's a really admirable trait from a comedy legend.