You're right that I misinterpreted it, but from reading the essay, it seems less like a substantive argument to me than dicking around with semantics. The whole point could have been made much more succinctly with a "taboo 'ought.'"

Any argument that entails responding to "you ought to do X" with "prove it" is awfully unlikely to convince your interlocutor; it's rude and will only set them on edge.