How is Bayesianism not a form of behaviorism? The essay devotes a paragraph to explain this point allthough Deutsch assumes the reader is familiar with behaviorism and probabilistic methods used i AI.
It was treating the human mind as a black box trying to pretend we have no insight in human thoughts. It was trying to avoid antropomorphizing people as Morgenbesser put it. In fact, quite the opposite, I'd attribute this blatant idiocy to Popperism. After all, introspection isn't testable or falsifiable in a popperian sense. There's no room for the concept of models in Popperianism. There are just big black box ideas that you test and are falsified or not.