You didn't mention if imaging studies were done to confirm a specific problem in the thoracic spine.
Assuming no injury was seen on X-ray or CT scan, the proof may be seen in the results of your treatment. If treating the thoracic spine improves your symptoms, then you either got better over time or the methods used made a difference.
Scientists studying the cause of symptoms with whiplash and the best treatment for this injury still aren't sure what works best. Some people seem to get better with treatment and some get better on their own while others have pain lasting months and years.
It's long been recognized by groups of chiropractors and some physical therapist that a whiplash injury has two places where the spine is "whipped" and "lashed" back and forth. First the head and neck snap backwards. The head and neck fulcrum over the middle of the upper back. Then the head and neck move forward again before returning to a midline position.
Even with the use of head rests in cars today, the middle of the upper back still takes much of the force of the injury. Treating just the neck may not resolve your symptoms. It seems reasonable that your team of health care specialists is treating this additional area.