Whilst people mention this external factor, that isn't entirely true in regards to PTSD, hence this push for injury is not entirely accurate by definition for external factors.
A trauma is required to meet the diagnostic criterion for PTSD, however; PTSD symptoms are part of hereditary mental illnesses, such as anxiety, mood disorders, depression and such. Trying to narrow these down to only being external is near impossible.
If you bite your nails, and have done so since a kid, then that is an early sign of hereditary anxiety. That means you have a biological aspect of PTSD within you prior to an external traumatic event taking place.
It would be impossible for the psychiatric community to go that route to "injury" until such time that they had actually empirically ruled out genetics in obtaining PTSD. Right now, that is not the case.
Those framing the argument to change disorder into injury, are only looking at what they want to view in order to back their argument, they aren't writing their argument or letters to include what the psychiatric community already knows, being there is to some degree possible genetics at play in PTSD to answer why some get it, some do not, when faced with the exact same traumatic event.
If genetics, cognitive and behavioural factors where not present in PTSD, then every single person when faced with a traumatic event would have the same signs and symptoms, which is not the case.
The website in question is framing one side of the story to suit their needs, not both sides, which provides the counter to their argument... which means it would stay as is until empirical data is present to outline exactly what PTSD is.
Post Traumatic Stress as a term already adequately frames a normal reaction to an external event, no question about it. The disorder is only added for those who don't recover in a timely fashion from it, thus identifying a deeper, personal, genetic, cognitive, behavioural and individualistic problem is preventing it, regardless of the external event itself.