From Previewing Android P:

In Android P we continue to refine Doze, App Standby, and Background Limits to further improve battery life; please be sure to try your apps with these and send feedback.

They don’t point to any specific improvements for Doze, App Standby and Background Limits. Maybe they didn’t quite reach their goals for these features. Or maybe developers and apps are not using these “good citizen” types of features enough to make the overall android experience better.