Paul, I was just as confused as you, but in the context of the paragraph, it makes sense. The preceding sentence added, it reads:

"The fact that our ethical intuitions have their roots in biology reveals that our efforts to ground ethics in religious conceptions of "moral duty" are misguided. Saving a drowning child is no more a moral duty than understanding a syllogism is a logical one."

The point appears to be that using the word duty adds too much conscious thought where there is none. Our selfish genes make us *lust* to save the child, regardless of how we justify it later that day while wearing a smoking jacket and scratching our beards. Similarly, it makes no sense to talk about syllogisms as "logical duties". You have to have understood them before you are even capable of having that discussion.