The comparison I referenced was with two different phones.
You have a point, but as I said, in my experience, battery Health is largely irrelevant, unless severely degraded.
I have another example. An iPhone 7, updated to iOS 12, lasts 4-5 hours. It was getting the exact same 8-9 hours on iOS 10, as my 6s is currently lasting.
I compared it directly back when both were new, the battery life was exactly the same. Played a YouTube video on the same network with the same settings, they dropped at exactly the same rate.
The 7 was updated, lasts 4-5 hours. My 6s was not, lasts the same 8-9 hours it always did. Is the 7 more degraded than my 6s? I don't know, but even if it was, it doesn't account for such a staggering difference. I tend to use devices for a long time, and battery life never really decreases. I never update. Is my battery health 50%? No, but I have had below 80% devices remain relatively unchanged (no large drops) in battery life, when maintained in the same iOS version. There is no way battery health accounts for a massive >50% drop in battery life, in my experience.