I'm guessing this is a gut feeling as opposed to any empirical data? Either way I'm curious as to why this would be the case. Unless I misunderstand the kernel is identical between LineageOS & whatever stock OS was on the device. And it's the kernel that presumably impacts most on battery consumption.
I installed LineageOS on an old Nexus 5. The stock OS on the Nexus is already pretty clean so I can't say I noticed a massive difference (although I didn't spend much time on it)
Also, installing SSL Packet Capture showed me that some of my (not even considered shady) apps made a lot of things in the background, eg sending stats or other data to their mothership.
What did do the thing for me was to go into battery settings and set the device in the lowest mode, that severely restricts background work. This, plus airplane, and the device basically doesn't drop anything over a night.
I miss some option (non-root since I want to be able to use bank apps) to tell my phone to never allow anything from apps A, B, and C unless it is active and/or in foreground.
Sorry if I sound like a broken record but to me run at boot, run in background, storage (outside your own not-shared-with-other-third-party-apps sandbox), and network should all be explicitly opt-in.
EDIT: Realised parent comment was about regular LineageOS, not the microG fork, never mind
This is completely anecdotal however, and it's likely that the reduced battery is down to having installed different/fewer apps on Lineage.
I find it infuriating when it's obvious some app is draining my phone's battery fast, but I can't see which, because Google is allowing developers to hide that activity within those two generic categories.