Why can't stress pronouns be used in the singular?

Question

Accepted Answer

Bonjour Chris,
Sorry for the late response. It can't be singular because they are two different people talking.
John: Nous sommes arrivés ! (Linda and I)
Alice: Oh non, pas vous ! (not the two of you!)

Answer

The question was "Nous sommes arrivés! - Oh non, pas ___! (We've arrived! - Oh no, not you!)" I answered with "Toi" (ie in the singular) but this was marked as incorrect with "Vous" given as the only right answer. If you could explain the problem with Toi, I would greatly appreciate it. Thanks

Answer

I have to say that it wasn't at all clear that the "oh non, pas..." was a reply to the people saying "nous..." I had thought it was a continuation - hence my frustration.