Yeah, I was a little sad reading this.
But I just dug into the specs for my Series 3 and it's only advertised as "18+ hours" also, yet I get 23+ hours from it even after a year and a half of true "all day" use. I wear it for sleep tracking with the AutoSleep app. It's in its cradle 20 mins twice a day when I shower. So yay for conservative Apple specs?
Hopefully this means the Series 5 isn't actually a battery downgrade. If it is, then the Apple-approved sleep apps in their App Store may not get the love they have received with older Apple Watchen.