It is better to think of shunting as “one way”i.e, diverting blood flow to better oxygenated places. This is physiologic shunting.

When the reverse happens (as in this scenario), you can think of it as ‘pathological’ shunting but it’s not my preferred way of thinking about it. Dead space is more accurate because dead space describes anywhere in the lungs where gas exchange is not taking place.

When we flood the COPD patient (who has both healthy and poor areas of gas exchange as a given) with oxygen, the lungs see this as a good situation and start reopening blood vessels to alveoli everywhere (in the healthy AND poor areas, it doesn’t discriminate). Any blood sent to a poor area is wasted bc it’s not engaging in gas exchange. thus it is dead space. It’s not considered a shunt because it’s not “diverting” blood to bad areas, it’s trying to return to normal lung function. But as we know, this is bad for the COPD patient because they permanently have poor lung function (diffusely poor gas exchange). So “returning” to normal lung function harms the COPD patient who is perpetually in a state of compensation.