I have a hard time buying this argument. If you are an employer despicable enough to systematically be paying women less just for being women, I don't think another redundant law is going to change your behavior.
This seems like a feel good exercise. People still buy drugs? Pass another law that says trafficking drugs is super duper illegal. Another heinous murder? Make it extra illegal to murder someone. In fact, make it extra illegal to murder someone with a specific type of weapon.
This stuff doesn't seem to matter. Drug dealers know they face stiff penalties if caught. People who have decided to murder another human really don't care if the penalties are higher if they use a gun versus a tire iron. And, misogynist employers aren't going to be swayed by yet another law that says treat all people the same regardless of sex, for reals this time.