I wrote a whole theory on this topic, and it makes is a challenge prove now that Semirhage is dead.

Moridin says that Sammael is alive, and Demandred immediately says that's nonsense because Sammael liked to be the center of attention. When Semirhage probes about the attack, Moridin says that it was Sammael who sent the Trollocs. At this point, Demandred realized that he'd made a huge error because he already knew about Semirhage's plan to attack Rand - he didn't know that she'd used a disguise in the form of Sammael. He therefore said he'd been wrong, to try and correct the situation, and it caused a stir.