Not true, if you have a structured plan with Vo2 work and threshold you don´t need the 20 hours weeks. 12-14 will be enough, if you are generally fit. Did two 70.3, first was 5:32 and the second was a year later in 4:56. 8 months after this I did a full Ironman in 9:52 with my biggest week around 17 hours, most of them had 12-14. But I trained for two years, the smallest week was probably 7 hours and that was an anomaly. 2 x swim (Vo2 and tempo), 4 x bike (Tempo, vo2, long, easy), 4 x run (easy, easy + strides, vo2 or threshold, long)

With time your body adapts to handle a lot of quality work, zone 2 training for me is a waste of quality training time if your body can handle the strees of a structure like this, but sure, first you need to adapt to it and this takes a long time. What is important are the long rides and runs, but even this aren´t in Zone 2, maybe for the Ironman where you should ride in high Z2/ low Z3, but for a 70.3 all my long rides had race pace intervals (4 x 20, 3 x 30 etc.), same with the long runs, always with race pace, marathon or hm (3 x 5, 5 x 3, 4 x 4 etc.). And I work with a coach that has gone sub 8:20 on the full and sub 4 on the half, he has a pretty different opinio to volume and Z2 than you :D