What is true is that organisms tend to act according to ethical fitnessism, which is what I stated. It is true by definition.
This might be a language issue, but no, this is not true because it flips the causation.
Saying that A (organisms) tend to act according to B (ethical fitnessism) implies that B came first and is the cause of A's behaviour. This is not true in this case. Here A's behaviour came first and you just stuck a label on it which says "B".
The sentence:
What is true is that organisms tend to act according to ethical fitnessism, which is what I stated.
does not imply any causation.
Natural selection favours certain behaviour, and ethical fitnessism is simply defined as:
…the ethic whose behaviour tends to be maximized as a consequence of natural selection.
Which behaviour that is is an open scientific question. There is no claim that ethical fitnessism causes organisms to perform any behaviour; natural selection is the cause.
I noticed that there has been some earlier discussion about Sam Harris’s Moral Landscape Challenge here at LW. As a writer on the Swedish politico-philosophical blog The Inverted Fable of Reality, I would like to share a response to the challenge, written by our main contributor, which I believe is interesting to read even if you are not familiar with The Moral Landscape or its content. See this link for the response and a short explanation of the challenge.
The response takes a different approach to most responses to the challenge. It is divided into four parts and starts by asking which ethic is most compatible with science and reality and finally tries to answer this question.