This is an interesting question and I like to find meaning in words, or what they mean to me.
I think PTSD can be described as a disorder, because I recognise that my thoughts are disordered. For example, what happened 5 years ago or fifteen years ago should be in chronological order and no longer bother me. But those experiences reccur in the present, and I want to put them back in the year that they happened - in their proper order.
But, the acceptance that it's not that simple - however much I try and however much I understand, I can't simply take a feeling I'm experiencing now, and put it back in 2006 or whenever. I have tried, and I havn't succeeded. So part of accepting that I can't control part of me (my mind), without medical intervention, it helps to view it as an illness.
I don't understand the word syndrome in any useful way. And injury, for me, is overly related to physical injury. I think PTSD is only really recognised after the initial injury has healed.