I think that the point is that sign is so closely associated with what it signifies that it is valid to speak of it as though it is the thing itself. Sacramental language. “This is my body” and “baptism saves you.” There is real grace communicated in these things also, but we would not argue that water or getting wet justifies, but baptism is a sign of justification and the righteousness of Christ and the idea of an unbaptized Christian is oxymoronic (though one is justified by faith alone).