They know.
At one point an
offer was made to Iran: just send your uranium to Turkey, and France and Russia will exchange it for ready-made fuel rods in the same quantity, for free. In return we will end the sanctions.
Iran turned this offer down. So, it was game on at that point. So now China and Russia are playing chess. Maybe they want a different balance of power. Maybe they figure Iran is rational, and will test their first weapon in a bunker in their mountains, and not in Tel Aviv.