Well, you make his point eloquently.
The holocaust was conducted clearly by an advanced state, signatory to many treaties and international obligations and "laws", none of which served to make any difference whatsoever, when that state decided they didn't care what the rest of the world thought.
But why stop there? Rwanda, Stalin's purges, China's Cultural Revolution, Kashmir, Iraq-Iran, until the U.S. got actively involved, all the U.S. wars against brown people, etc., etc., etc. When has international law, regulations, or even opinion, ever changed the conduct of an aggressor nation when they decided to go to war? The reason nukes haven't been used since Nagasaki is only because everyone who has them is afraid if they used them in aggression, it would trigger a much higher escalation, and has nothing to do with any treaties, laws, or world opinions.