I think I've always assumed it meant "unimpressed." That is not the "new" meaning under discussion, but they seem to hint at it when they say, "This new sense appears to stem from a mistaken belief that the first three letters of nonplus are there to indicate that someone is something other than “plussed” (although what being plussed would entail here remains a mystery)."
I bet my meaning is the next change to this silly word. :)
I have never known it to mean anything except for "perplexed", as a hiberno-English speaker. But now that I do know the newer meaning I'm both mildly nonplussed and totally nonplussed about it.
And this reaction, I think, hint at why this shift has happened: It will often be unclear if someone is unfamiliar with the word, whether it means perplexed or unruffled, because often the same situation would justify either.
You might be perplexed at the reason someone cares about a situation because you yourself is totally unruffled - being both nonplussed and nonplussed about it... At least a couple of the examples they give are ones where either meaning is plausible.
And so if someone is unfamiliar with the word, it'd be easy for them to infer the wrong thing and as a result associate the wrong meaning for the word going forward.
"I looked at Jim and he seemed nonplussed by the situation." Is Jim acting cool and relaxed according to his character, or is he uncharacteristically flustered? The author knows, the reader might not.
Also a hiberno-english speaker, I've always assumed unbothered to be the primary meaning. I was vaguely aware of the autoantonymic usage but definitely felt less common.
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.
> "unimpressed." That is not the "new" meaning under discussion
That is pretty much (very close to) the new meaning under discussion: as the article says, the old meaning was “at a loss as to what to say, think, or do”, and the new meaning (started showing up in the early 20th century, though I only encountered it recently) is “unruffled, unconcerned”, which is close to your “unimpressed” (and close to the opposite of the earlier/standard meaning).
(The upshot is that the word “nonplussed” is basically skunked now, and should not be used because readers will misunderstand/be unsure. Some discussion in this thread https://mathstodon.xyz/@dpiponi/111684566418809307 including examples of “enervated” and “livid”, and the observation that the etymology of “non plus” is similar to “I can't even”.)
The context in which a word is used is typically more informative than the meaning in the dictionary. For example, "set" has an unreasonably large number of definitions [0] but I can't remember the last time its usage in a sentence was confusing.
It's also why "cromulent" from The Simpsons had a clear meaning during the episode that coined the word even though it did not exist prior to the episode airing.
That's how we learn 95% of words' meanings: by osmosis from hearing them used by others whom we presume know their proper meanings. I doubt you've done 100,000 dictionary look-ups, or any number remotely in the ballpark of the number of English words you know.
I bet my meaning is the next change to this silly word. :)