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).
We probably won't witness a spectacle like the one with The Oatmeal again in our lifetime! Yes, the title is needlessly sensationalist but it astounds me how we can't even get media professionals to get these things right. Would TNW be very accommodating to people who share their content this way?
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).