Laws like these are not criminal laws, and are not meant to be enforced if by that you mean policed. It's up to someone to file a lawsuit, if they think the law is on their side. Then a judge will have to rule whether or not the law applies in that particular case, and in doing so they may set a precedent which will influence how later cases will be judged.
And following that further along, in this way, I can see a pretty straightforward method of declaring any job advertising as discriminatory under that decision
You may think so, but this is why we have judges. Just because you can make and argument, it doesn't mean that argument will be persuasive to a judge. You may further think that this means that the outcome of a trial is arbitrary, and you would be right in the sense that law is not an exact science, and the result of a verdict can be difficult to predict if the evidence is weak and/or there is no precedent. But you would also be wrong in the sense that each verdict rendered helps to set precedent, and judges (and lawyers) always look to see what precedent has been set, both when it comes to how to interpret the law, but also when it comes to establishing what counts and doesn't count as evidence.
Disclaimer: IANAL. I'm not even American.