Is it really accurate to assume PTSD is the result of prolonged stress though? As an example I would point to rape victims or people who've been in car accidents. Those are singular sources of very intense trauma that seem to overwhelm the brain.
I would argue that such cases had to have been very prevalent after ancient battles, but they were undiagnosed or labeled other things (e.g. cowardice). I know there was a culture glorifying war and they experienced death much more than we do today but our underlying psyche is the same and subject to the same vulnerabilities.
So the question for me isn't whether or not PTSD existed, I think it clearly had to have. The question of interest is really how they coped with it and how society reacted to sufferers of it.