The original experiment was four days per week of the "exhaustive intermittent training" and a fifth day was 30 minutes of zone 2. That's what I followed. It does seem like that fifth day has been forgotten when people talk about Tabata. Like you said, "zone 2 training and HIIT/Tabata are not mutually exclusive," and I've gotten my best results when doing a few hours per week of zone 2 running with a dash of higher-speed intervals or repeats one day.

Now that I'm quite a bit older, despite maintaining my body weight and two-mile running time since then, I'd probably get hurt if I repeated the experiment.