People can have physiological reactions to all kinds of things. Up to, and including full on panic attacks and worse. Both real & imagined.
The whole, there isn’t any symptom of PTSD that’s unique to PTSD. They’re all shared with other disorders, as well as happen to people with no disorder whatsoever. In point of fact the foundation of several different disorders is that they’re having physiological reactions to things that aren’t real, and never have been. Delusional disorders, OCD, Phobias, just to name a few. But even a new parent in terror of their child being killed, or a betrayed spouse sick over the image in their mind? Happen. Rather a lot, actually.
The mind is a powerful thing, and the brain is quirky as hell.
That people with PTSD tend to use whether or not there’s a physiological reaction or not as a guideline to what’s real, doesn’t mean that someone with GAD or Claustrophobia can do the same thing. In fact, for other disorders, a physiological reaction is often used as a clue that it’s NOT real.