I have absolutely no idea what specifically this is in reference to (other than some criticism of Secret Six), but I agree with your judgment of the comment on general principle.
I mean, “include nothing that does not…
Huh. Do people generally mean just plot when they say that? I thought it was more like… where writers repeat stuff that the reader already knows/can already infer from the situation; or else to showcase how much research they’ve done, in ways that don’t add to sense of place or character, rather than telling the damn story.
I’m not a linear or plot-driven writer at all, and often need help with that side, but I quite often feel that work I’m reading/critting could stand to lose repetition of the obvious or irrelevant.
I may be entirely missing the point though, brain not braining so much atm.
I couldn’t speak to what people generally mean when they say “It adds nothing to the story.” My experience with it is that it seems to come up most often as a knee-jerk judgment on the value of things that don’t advance the plot or the themes a critic has picked out as important. A person who spends more time involved in things like workshops, teaching, constructive criticism circles, or writer-editor collaborations might tend to encounter a more nuanced employment of the phrase.
There are situations in which “This adds nothing to the story.” could be sound criticism or the basis of good advice. My problem is with people who have internalized the idea of that sort of thing as good advice/criticism to give, regardless of circumstances and the actual goals… like someone who had good luck with turning left once, and so now peppers all the directions they give with “turn left” on the idea that it’s a sound principle worth following at all times.