What she meant (I think it's safe to assume) was something along the lines of...

  While it's obvious to you and I standing here examining it that
  it's a prop gun, if it were being wielded in a threatening
  manner, it would be far less obvious, and it's reasonable to
  assume that it could be perceived as a real threat.

Given the fact that there are several models of gun that small that are quite capable of killing you, that seems like a pretty reasonable argument to me.