Thoracic Extension is a passive result of the active movement of the hip to trunk relationship, so yes, in a sense, it is a result of the hips firing and the trunk turning back against the hips.
Also, efficient arm action will allow for a more natural thoracic extension to occur during the throw, it won't be forced. Max external rotation can also be achieved from max thoracic extension and of course this hip to trunk relationship.
Structurally, T-Spine extension and mobility can be increased but make sure that stability is not lost during this.
My book on High Level Throwing Patterns is almost done
Also, efficient arm action will allow for a more natural thoracic extension to occur during the throw, it won't be forced. Max external rotation can also be achieved from max thoracic extension and of course this hip to trunk relationship.
Structurally, T-Spine extension and mobility can be increased but make sure that stability is not lost during this.
My book on High Level Throwing Patterns is almost done