The whole thing sounds like blatant discrimination, and I'm guessing the employer didn't even think about this, or assumed it didn't apply to a 'casual' advert? They obviously don't know much about labour law either, or they'd never have sent the reply the OP described. We all know that employers have various ways and means of getting the employee they want, and it's hard to prove that they broke the law in most cases.
I suppose you could go after them, but I doubt if it's worth the trouble.
I suppose you could go after them, but I doubt if it's worth the trouble.