And why would Sammael tell a lie that they could disprove without his help? All they would have had to do is induce Rand to swear and prove him wrong, which would be fatal to his hopes to use them to get control over Rand for his purposes. If he did think it might work on Rand, he would never have given them the Rod for the same reason Graendal preemptively refuses to train Someryn. At the least he would have hedged it with BS proscriptions and conditions like he told them for the "nar'baha." He could have told them something like "the feedback/backlash will kill anyone nearby" or something similar to dissuade them from trying it. He would not have chanced Rand ending up in the control of others who had ways of getting in touch with him.