It means that the Dark One doesn't think as humans we do; It doesn't understand our values, motivations or mindsets, but it can be entertained by and have an appreciation for the excesses some of the Forsaken exhibit. The Forsaken that seem to stand highest in it's estimation, always included Ishamael/Moridin, Lanfear, Graendal and Semirhage and part of the reason is probably because they entertain the DO the most with their attempts to get and keep his attention.

Just because the first thing you would do would be to annihilate Semirhage doesn't mean that this is the first thing the Dark One expected Rand to do. I think the Dark One actually thought Rand would make contact with the TP, be amazed at how powerful he could be through Shai'tan and be convinced to follow him in a way that Moridin/Ishamael never did.