The duty still exists, regardless of your
purpose or
audience, or whether it's
convenient for you to say "and btw, this is from XKCD"; however it's rather unlikely that anyone would try to push you to uphold it in those circumstances.
The claim here seems to be "as Editor-In-Chief of a site dealing eminently with the Web, he should know and follow it"; but indeed - trying to stir up a storm in this very tiny teacup sounds forced and sensationalist (there are much more interesting cases, such as The Oatmeal versus that "fun-something" site).