This fits with what I've seen. Got the new 15" a few weeks ago, and transferred my old system over to it. Battery life was 5 hours at best, usually less (actual usage, not estimated). But then just a day or two ago I'm suddenly getting 8-9 hours with no problem, no change in usage or applications. I don't have much data in iCloud either, as far as I know.
If it is related to background indexing, I'm surprised that there doesn't seem to be much visibility into it. None of the Activity Monitor tabs or tools such as htop revealed any particular heavy CPU/disk usage to which I could attribute the poor battery life.