That is correct, or at least this is the philosophy I am adopting for the last two years when I embarked on TAO training. Volume is time base rather than distance base.. intensity is pace base rather than HR base.
Idea is you spend a certain duration in that zone so regardless if you are elite or average runner, your TRIMP score is the same. I.e in singlish, the siongness is the same for the workout. Hence muscle, skeletal and aerobic gain from the training should relatively be the same proportion.
If it's distance base, the average runner in the same intensity zones as the elite runner will have a harder time, coz he spend a longer time is that same intensity zone. i.e his trimp score is higher than the elite and a supposedly same workout ended up more siong than the elite.
But my advise is to use pace base training intensity zone when you have a target pace for a race. Else for self diy with no race goal , I prefer HR intensity zoning.