I'm not picking an argument here, I'm just curious about how this works ...
What's the problem with zone 3 in this case? It's aerobic so surely time spent in zone 3 during a long ride is not lost - is it not contributing your endurance training just as zone 2 time is? Is the issue with this theoretical cyclist not that they are riding in zone 3, but that they are not riding enough hours in whatever zone? Because their zone 3 rides are shorter duration harder rides than they would be if they chilled out and rode for longer in zone 2.
I don't have a direct interest in this in that I never have a training plan of any kind. I just like riding my bike, but I also really like stats. So I enjoy retrospective analysis, but don't prescribe how I should ride. So when I look at one of my long rides in retrospect I see generally a roughly even split between HR zones 2 and 3 with a bias to 3 with (much) more 4&5 if it was a (very) hilly ride, bias to 2 if it was flat. The ratios are generally dictated by the terrain and weather, and sometimes external events like train times. On the road I don't really have the mental discipline or desire to do anything other than ride my bike how I always ride, so if I wanted mostly zone 2 I'd have to plan a flat route.