I'd disagree here, not because I agree with the concept of genocide, but I've always felt that was one major trade-off of free speech, was the responsibility of the people that comes with it. While it may be advocating it, no one is explicitly stating they will do it, meaning, while the idea is bigoted and stupid, the speech itself isn't illegal. The reason free speech exists is so that every idea and criticism can be heard. That said, I've always felt the most important part of the listener is to analyze what was said for themselves.
Can the speaker be arrested? No. But the people who have heard what they have said have a responsibility to react accordingly. If what the person has said is reacted to, this leads not only to destroyed reputation and controversy, but can damage their business/get them fired from their workplace, as well as get them banned from certain privately owned buildings.
Bear in mind, just because something is legal, doesn't mean it doesn't have consequences. Freedom of expression goes beyond speech, and should you have a dislike for something that has been said, then you have the right, and sometimes even the responsibility to make that known through your own expression.