Adding multiple dust specks to the same people definitely removes the linear character of the dust speck harm-- if you take the number of dust specks necessary to make someone blind and spread them out to a lot more people you drastically reduce the total harm. So that is not an appropriate way of reformulating the question. You are correct that the specks are the "wrong answer" as far as the author is concerned.
Did the people choosing "specks" ask whether the persons in question would have suffered other dust specks (or sneezes, hiccups, stubbed toes, etc) immediately previous by potentially other agents deciding as they did, when they chose "specks"?
Today's post, Torture vs. Dust Specks was originally published on 30 October 2007. A summary (taken from the LW wiki):
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