Are you thinking that Kincaid actually /is/ human somehow, rather than simply lying to Harry that time? Id always thought His unconscious Choice had to be his Inhuman side which is what kept him alive. Unless he chose Human and his Human side turned out to be a Wizard? That only works if he's somehow Lost his magic recently (otherwise he'd have used it in SmF, no?).
I don't think Kincaid was lying. I think he thinks he's just as human as Harry, who isn't entirely human. Compared to vanilla humans, Harry is a demigod. He'll live for centuries, his injuries heal much better than normal for humans, and he has access to power that lets him manipulate reality around him to certain degrees.
By comparison, we know that Kincaid has lived for centuries, has somehow survived the things he's faced in that time, and has senses (and potentially abilities and skillsets) beyond human levels. He could quite possibly be more human than Harry in some ways.
Now, as to the source of Kincaid's longevity and senses, I'm not sure. I don't think we know enough about the nature of Changelings/Scions/Nephilim to know what physically and spiritually happens to them if the choose mortality. Are Kincaid's genetics superior to a normal human's because of his parentage? And if so, is his physical body left as-is if he decides to remain mortal? Maybe, maybe not.
Personally, I figure wizards are descendants of scions that chose mortality, but got to keep a small measure of power or ability. Kincaid might be the same, but from a different branch in the sire-tree.
Which would make him just as human as Harry.
