That meaning is actually already in webster, together with "not surprised, not bothered" which is probably what the articled describes as unruffled [1].
Not sure why the article pretends like they haven't already added the "new" meaning to their dictionary. Maybe it happened after the article came out
"NOTE: The use of nonplussed to mean 'unimpressed' is an Americanism that has become increasingly common in recent decades and now appears frequently in published writing. It apparently arose from confusion over the meaning of nonplussed in ambiguous contexts, and it continues to be widely regarded as an error."
However the article explicitly says "we’d just like to give you fair warning in case our descriptivist nature causes us to take action" which implies that they hadn't actually taken added that meaning at the time of writing.
Not sure why the article pretends like they haven't already added the "new" meaning to their dictionary. Maybe it happened after the article came out
1: https://www.merriam-webster.com/dictionary/nonplussed