Saying it's encoding human irrationality is taking the viewpoint that the human reaction to the fat-man trolley problem is an error of reasoning, where the particular machinery humans use to decide what to do gives an answer that does not maximize human values.

It makes some sense to say that a human is a holistic entity that can't be divided into "values" and "algorithms". I argued that point in "Only humans can have human values". But taking that view, together with the view that you should cling to human values, means you can't be a transhumanist. You can't talk about improving humans, because implementing human values comes down to being human. Any "improvement" to human reasoning means giving different answers, which means getting "wrong" answers. And you can't have a site like LessWrong, that talks about how to avoid errors that humans systematically make - because, like in the trolley problem case, you must claim they aren't errors, they're value judgements.