He could be talking quite literally or at least as literally as a Fallen Angel can. I'm reminded of Harry's conversation with Lash.
"I mean no insult by it, but you should know that your abiolity to comprehend your environment is very strongly defined by your belief in a number of illusions. Time. Truth. Love. That kind of thing. It isn't your fault, of course-but it does impose limits upon your ability to percieve and understand some matters"
"I'm only human," I said. "So enlighten me."
"To do so, you would have to release your hold on mortality."
I blinked and said, "I'd have to die?"
"Again you have only a partial understanding. But in the interest of expediency yes. You would have cease living."
Harry has died and has learnt a lot about Time and Truth since doing so, but alas that's a different topic and conversation, one which as some people have noted has been Beaten like a Dead Horse.
I think Nicodemus is being as literal as he can be. Harry is exhibiting too much emotion, too much self awareness, too much independent thought regardless of outside influence to be Nemfected. Basically he's being too much like himself and too human. If he was corrupted, he wouldn't be so much like himself he'd be playing word games and subterfuge who as others have pointed out in this conversation is brash and upfront.
So like Lash, Nicodemus is giving him a reverse complement and left handed insult y saying that Harry is still too human to have been influenced.
I wonder if his having dying and being exposed to those subjects since also makes him more vulnerable to Nemfected just like he's vulnerable too his Mantles. (Everything relates back to Mantles)