The idea of a dog whistle is as you say only dogs hear it. So in the use case of Nazis letting each other know they are Nazis it’s usually a subtle thing. A phrase or gesture that’s innocent on the face of it, but signals to other Nazis they are Nazis. Like what they tried to do with the OK hand signal. Putting up your hand and shouting Heil Hitler isn’t really a dog whistle, as everyone knows at that point you are a Nazi.