is that applying his rule means anyone who says “Israel should stop existing and Palestine should be the only state in that territory” is out of line

I think his point is that it’s out of line if the same person also calls for respect of international law, because that’s an inconsistent position (international law supports Israel’s right to exist, so calling to respect it means also calling for that). As I said before, I think international law is already worthless, but I see the logic of the argument even if I disagree with it. I might have been misremembering his position, though.