This is good advice and I always have a digital copy available and paper ones when I visit but given the time/distance if there was such an emergency I wouldn't be there to ask.Capacity related to specific decisions so there isn't a blanket assessment to be made. Presumably your GP thought he had capacity for something important but as you point out things fluctate so the GPs advice could be out of date. Dementia only gets worse. If you were in a position where resuscitation was being considered it would be obvious that he did not have capacity then so you would be able to make decisions. It would be as well to make sure you can easily produce the POA should the need ever arise.
I was also making the point that capacity is not a blanket decision so am still not sure what @Ferrit44 is having a problem with.