Matrim not only has the memories of the great dead war leaders but is actually them reborn. The wheel only puts new strands in every now and then and Matrim's is not new, just like Rand is the Dragon Reborn!A huge clue is when in the First Book Matrim comments to Tam "Battles interest me" (TEoTW, p54). And Mat already had a knack for speaking the old tounge beforehand as well, he wasn't just given these memories, they were his own from past lives!