Hey Matt!
The exact heart rate/metabolic zone for these sessions will vary a bit between individuals. Caleb is extremely aerobically fit, so his heart rate for these sessions often drifted from below his aerobic threshold to a bit above for these sessions. Someone with less aerobic fitness may do these reps all above aerobic threshold.
If you’re trying to replicate these sessions, I recommend using perceived exertion as your guide. Minimum work for this session should be 30 minutes, max around 70 minutes (for very well trained individuals). Working reps should be 5-12 minutes, rest intervals 1-2 minutes. Shorter for shorter intervals, longer for longer intervals. You should carry a bit of fatigue between reps. The length of the intervals matters little, the goal is just to accumulate time at intensity. The workout should feel hard, but you shouldn’t fade (Ie your paces should be even across the session or getting slightly faster) and you should finish the workout with one to two additional reps in the tank.
Let me know if you have any questions,
Jack