I don't think that particular theory is valid. For example when an old person dies it might be that they have cancer but pneumonia kills them, they have a weak heart but influenza actually delivers the coup de grace. With the best will in the world someone who dies when infected with COVID could actually meet their maker via heart failure, respiratory problems or even dehydration. So was it COVID that was the cause of death or heart failure? Without COVID, they live on and without the weak heart too. Impossible to say which in many/most cases.
Therefore, I think that excess deaths show a pretty reliable measure of pandemic mortality.