I figured they would learn by getting called out like this. And since the post was anonymous, it doesn't really hurt them anyway. Free learning experience.
There have been previous discussions about the state of some YC job ads (though I can't find them right now), so this isn't the first time people have felt compelled to say something.
edit: Link to previous job post discussions (I know there are others but can't dig them up).
Another option would be to simply email the given address and state your complaints. For example that you may have been interested (if you were), but felt mislead by the posting.
It's interesting to think about my own motivations -- I wonder if I would have been gentler if their post hadn't been anonymous. If so, should I have been gentler even though their post was anonymous? Anyway, I apologize if this caused unnecessary hurt, I didn't mean to.