The fact that they're having a conversation on this subject at this particular time and place is not the problem. The problem lies, first, in the combination of that fact with Lyra's being conveniently present to overhear them, which strains the credibility factor, and, second, in the stiff and expository nature of the dialogue, as if they're thinking to themselves, "We must talk in this overexplained way so that Lyra will understand what she's overhearing." If they had been deliberately talking that way for that reason, it couldn't be worse.