Nope... disagree with the self diagnosis, however; I would not consider students assessment as a diagnosis, so in your case, I can see your point, but the moment you went to someone with actual experience and qualification, they got you right. Saying that... students also tend to diagnose by the book, so they more than likely got it right by the book, but if you are constantly seeing a different person each time, that is where the issue lay, because an actual diagnosis for PTSD should only be given after 3 - 5 contact hours (from memory) with the same physician (psychiatrist / psychologist).
Even going to a professional with experience, something like PTSD should not be the first diagnosis explored, as mental health doctrine is a process. Some cases a trained eye can just go, yep, PTSD... but if that is not the case, then it must be an evaluation process and even trial and error, as PTSD is the last option on the list pretty much.
Sorry... but I disagree that any online assessment tool for PTSD, and most mental health disorders actually, could be close to accurate. Gauging symptoms is only half of a PTSD diagnosis, and that is all an online assessment focuses upon, symptoms and whether someone believes they have endured an event of significance to meet criterion A. Online assessments are pretty much giving people the misrepresentation of self diagnosis.