I had some problems with terminology like this a few months back. I was very distressed by the idea that I might have "a mental illness." On the other hand, I have no problem accepting that I have suffered from clinical depression, which is no doubt a mental illness. Something about PTSD really bothered me.
I think, honestly, my frustration was in being "stuck" or "labelled" with a disorder based on something that happened to me. It pissed me off; it wasn't fair. I've since chilled on that point. There are things about the way I am that are no doubt a bit off the norm, things that are explained by the diagnosis of PTSD.
Here's the question: what if PTSD is a psychiatric disorder? Why is that important to you?