There is a famous (among exercise physiologists anyway) line from Per Aastrand’s bible of exercise physiology printed in 1970 that says:
It is an important but unresolved question: Which type of training is most effective for building endurance? 1) To maintain an intensity level representing 90% of the maximum oxygen uptake for 40 minutes. Or 2) to tax 100% of the oxygen uptake for 16 minutes.
This does not exactly answer your question but you can see that there is no one answer. For less fit athletes Z3 intervals will be useful because they will not be able to maintain the same output for 20-40 minutes. But fitter athletes will benefit from longer sustained Z3 efforts.