From Blood Rites, pages 94-95:
You were saying?
Thanks for the quote. I should have done so.
He is clearly not under a fairy like truth compulsion. Reading this it feels like a very human reaction. It is like asking Ramirez about his sex life.
Some questions are very personal. Asking a half human entity what he is is like asking him how he regards himself. He may have wrestled with the question in the past. He may even be lying to himself. And anyway it is none of your bussiness and you should just not ask a question like that.
Don't ask. Don't tell. And if you do ask you can expect a lie.
In a normal human sense it may not even be a lie. If they both know it is not true and know the other knows (and so on) it stops being a lie and is just a way of telling the other not to ask questions.
Of course Harry ignores this.
In reaction Kinkaid does not even try to be convincing. He just makes fun of Harry.
Compare with how Harry in small favor reacted when Sonya wanted to know what Gard was.