Why does the Houyhnhnm laugh that a "Creature pretending to Reason, should value itself upon the Knowledge of other People's Conjectures.."

He laughs because these things (endless talk of reason) are merely vain ramblings of men with too much time on their hands, and if what they believed did match up with the rest of the world....... it would be bad. This is the reason the HOUYHNHNMS aren't consistently dealing with disputes and controversy.