I don't get the reference

The reference is to “Fenimore Cooper’s Literary Offenses” a takedown Mark Twain, considered by some still one of the most savage literary criticisms ever published, made about Fenimore Cooper’s book “The Pathfinder, or The Inland Sea” where among other offenses (literary and otherwise) a character proves a “good marksmanship” by hitting the exact same bullet hole as a first and then the main character proves to be even better by doing it a second time (a third bullet through “the exact same” bullet hole. (Among other things, multiple bullets through the “same” hole would expand the hole making it easier to hit, not harder. Which also relates to the fallacy of the given tests being additive. Presumably if the shot is lined up to be fair it is easier than the sum of its parts.)