Then why does supplemental o2 not work in low v/q if blood is moved from poorly ventilated area to well ventilated area?I think you might be misreading some of the language in the question. I'm sure it was trying to say there was NOT a ventilation defect. That's the diagnostic criteria for PE on a VQ scan. I think how they meant it was as a perfusion defect that has not been matched.
This gets at a key piece of lung physiology: the lungs are the only organ that will vasoCONSTRICT in response to hypoxia (aka ventilation defect). So lets imagine we had a foreign body somewhere in the bronchial tree; the VQ scan in that case would show a ventilation defect distal to the obstruction AND a perfusion defect in that same area.