Maybe unsurprisingly, Garmin is overstating their case. I've been wearing a Fenix 6 Pro for the last year, and a FR935 for 2 years before that. I play golf 3 days a week, go to the gym 2-3x a week and mix in a couple outdoor walks/runs. Wearing the Fenix 24/7, I have to recharge it at least once a week, sometimes every 5 days. Definitely better battery life than the Apple Watch, but not "months". Not even "weekS" (plural).
I can get two weeks out of it if I just wear it as a watch and don't do any workouts that require GPS, and I could maybe get a month if I put it in "Expedition Mode", but that disables the screen, most of the sensors, and only takes a GPS reading once an hour. At that point you can barely even call it a "smart watch".
I have my feet in both camps. Garmin doesn't make bad products by any means, and there are certain use cases in which Apple doesn't match up - but Apple offers other features that Garmin doesn't come close to. Battery life isn't one of them, but Garmin's isn't as good in real-life use as they're crowing about.
When my AW8 arrives next week, I'll probably keep the Fenix as my golf watch and use the AW for everything else. Garmin's native golf app is as good as any AW golf app, doesn't cost me anything extra, and sends all my rounds to the Garmin Golf app for tracking and analysis. And I don't have any worries about the battery lasting if the course is crowded and I get stuck in a 5-hour round of golf.