Well all right, Harry. I prefer the full OED but I agree. A nonplussed person is doubtless surprised, but a surprised person is not necessarily nonplussed. My quote from the SOED was intended to show the difference.
You're both right. The original meaning of the word is "surprised" but in more modern usage it has come to mean "unfazed; unperturbed." I was using it in the latter sense.