It's true that Chains work in Dungeons as well, but in the case of Dzemael Darkhold, due to the enemies being lvl44 you won't be chaining anything if you're above lvl44 yourself. Thus I'd imagine that's not it unless Lord Nick forgot mentioning his friends' levels in detail.

And well, about the 50% Exp bonus for levelling a new class, it's active as long as your highest class is a single level above it. There's no increases or decreases depending on how far off you are.

Did your friends have Rested bonus active perchance, while you didn't? That's another 50% extra EXP to enjoy per kill.