Limb muscles create movement, torso muscles create stability.
The simple act of walking requires spine stability. There are no agonists and antagonists in the torso they are all required to stabilise force generated elsewhere in the body.
If your therapist treats your core muscles in the same way as limb muscles, they are simply going to extend the pain in your back
Elite performers generate force in their hips, and it is transmitted through a stiffened core.
Spinal Control: The Rehabilitation of Back Pain: State of the art and science
edited by Paul W. Hodges, Jacek Cholewicki, Jaap H van Diee