Funny. In my head, nonplussed was near-synonymous with unimpressed. So in my interpretation they were neither not paying attention, nor in over their head -- but the blank stare remains nonetheless. That still closer to your interpretation than "baffled, perplexed" as it apparently originally meant.
I also, from reading a lot of literature, took this meaning. I’ve never been a fan of the word though, probably partially because of the ambiguity being discussed here
