What does it mean when an assertion “detracts from the author’s argument”?
You seem to choose rather tough reads – good girl!
Detract is the opposite of praise. So, when an assertion “detracts from the author’s argument”, it means that someone says something that is belittling what the author meant to say, in other words they are lessening the value of the author’s argument.
Detract from has the sense of taking something away from another thing so that it is made to look less effective. In your sentence the remark/assertion removes the reader’s interest from the main argument and in this way tends to devalue it.
So, if I understand correctly, if an assertion detracts least from the author’s argument, it means this assertion agrees most with the author’s point?
Well - I suppose so. A strange way of putting it. I think I need to know the whole context to give a considered opinion.