I mean Semirhage's whole plot becoming Anath and later posing as Tuon during the meeting with Rand was a big lie. This shows that Semirhage of course lies at length if it suits her plans. What Semirhage said in KoD, she clearly said just to cause dissension among Rand's followers and unsettle Rand. She can hardly know what is going on in Rand's mind. Even Graendal doesn't know it, but just guessed. We know that Rand isn't insane (yet) and therefore hears a voice of a past life (as Semirhage implies), but that LTT is really a personality in his mind, probably provided by the Wheel. It's also most questionable if this could abruptly lead to "terminal madness".