Making a point, a statement, is still starting something. It doesn't have to be a bad something. But all conversation starts from a point.
So it's better to be "not everyone knows ASL" and not add on that there are so many different kinds of sign language so that she knows better in the future? It's better not to say that it's wrong to assume when you're in another country? That it's even worse to say things like that? No wonder the human race gets so stuck. Too many of you fear the right criticism to learn by. No one wants to get corrected and realize their wrongs so it's better to just offer a tiny smack on the wrist. Spare the rod, spoil the child. Progress is not made with tiny smacks.