It looked to be a great run for Zach (9-min for so PR) and he worked hard for it (140mpw etc.)
I'd probably think his HR was at least in the 140s or not a super easy % of max, based on his age, level in training and ( and roughly 2:30 marathon PR).
That being said there is often a decent variation in max HR. even between runners of the same age/experience level in training. For example, I've seen data from Andy Wacker doing a 20-mile Long Run at the Res at about 5:20-5:25 pace and average a HR around 170. For me personally (a couple years older), I can hold 170 for maybe 40-60min at best but it's over my Lactate Threshold and it's going to be around 5:10-5:05 pace at altitude. But that's because my max is likely way lower than Andy's! At 7:00min pace here in Boulder I'm just under 130bpm. It's likely Zach probably has less "wiggle room" with the HR variation, but his issue was overcoming mounting skeletal muscular fatigue, burning a high % of fat as fuel, and staying hydrated while not getting any GI distress.
One thing I've learned doing ultras is that you can compete at what seems like a pretty high intensity for hours on end.
Road Marathoning is of course a bit more intense overall, but esp. in the high altitude mountain and hilly ultras you an literally run right up to your Lactate threshold at times as long as you back off on the downhills.
It would be hard to twist his arm to do this, but I'd be excited to see what someone like Max King could do for 100-miles on a track/flat road. Given his 2:14 marathon speed and experience at trail -100 milers as well as being 100km road champ, I'd be willing to bet he could lay down a pretty strong pace for 11 or so hours...on a flat runnable surface.
Congrats to Zach!