Inka-Reb is a powerful Dhillarerean, and when he sees Maerad he is able to perceive certain things about her that most others can't. And one of the things he sees is that Maerad is fiercely denying parts of herself, and ignorant of others (the ignorance is maybe the real reason why he won't speak to her, because he would think her dangerous; which she is). I think, in particular, that he sees that Maerad denies her ability to love (stop jumping up and down, Cadvan/Maerad fanatix
) And remember, Maerad has killed a Bard in anger, and hasn't dealt with the implications of that, either. Inka-Reb's evading Maerad's question slightly when he says all human beings are liars, though he does mean that all humans delude themselves.
It doesn't mean that the Elidhu have a monopoly on honesty, but they probably can't see the point of lying, unless they are thoroughly corrupted like the Landrost is by Sharma; though they might be deceitful in other ways. But that's probably more to do with human beings projecting things onto the Elidhu. Arkan has a few things to say about truth as well, and he certainly isn't lying; one of the troublng things about him!