It was probably good advice. Many of the conditions that cause pain in the mid back are either directly or indirectly related to your posture.
Poor posture can cause muscle imbalances and other physical changes in the body.
Muscle imbalances can make us more vulnerable to conditions such as postural syndrome, thoracic joint irritation, certain types of scoliosis and subluxations. But posture is not the only cause of mid back pain. Injuries due to trauma or repetitive movements are another source. In short, no one is immune from these injuries. From people who work at a desk to those who paint houses, anyone can become affected.