But that huge reduction in battery life is when you use GPS activity tracking. GPS usage on any watch will impact battery life, including the Apple Watch Ultra.
Comparing both watches in activity tracking mode + AOD off, the Garmin (44h) still has 2x the battery life compared to the Apple Watch Ultra (20h).
You also get a huge reduction when you turn on SpO2 (off by default on Garmin, on by default on iWatch). Same for music, always on display, etc etc etc.
My understanding is that no smartwatch takes SpO2 readings during activity/workout tracking as the sensor is not accurate when you are in motion.
The 20h number I mentioned is from the OP: "For continuous outdoor workout tracking, Apple Watch Ultra 3 now gets 20 hours of battery life in Low Power Mode with full GPS and heart rate readings." Low Power mode disables AOD and SpO2 (which is already disabled anyways during workout tracking).
I doubt SpO2 is very battery intensive either, certainly it can't be worse than the heart rate sensor. Both sensors work in the same way fundamentally.
Comparing both watches in activity tracking mode + AOD off, the Garmin (44h) still has 2x the battery life compared to the Apple Watch Ultra (20h).