The inyan of having the same voice I have heard is that both his parents teach him the same values, and he doesn’t get mixed msgs. Because nobody is ever brought up under optimal conditions, it is only a hypothetical, and never happens. Drosh vkabel schar.
My main question is that it seems to take a pessimistic view that some people have no hope. I believe I heard in the name of the Baal Shem Tov that the worst aveira in the whole Torah is to think one is too big of a sinner to do tshuvah. So I am not sure how that fits here. I prefer the Baal Shem’s viewpoint.