My Debian 12 VM has been up for 4 days, and Xorg clocked 5 hours of CPU activity in that time, but systemd clocked only 9.5 seconds, and its subsystems each less than that.
So I doubt if systemd and its subsystems are using much energy.
It doesn't seem likely to explain 20 vs 8 hours battery.
Its less the cpu usage, but more the frequency of wakeups.
Could probably get similar improvements by using any distro with less "batteries included" than Debian. Though I'm not aware of any except Alpine which offer stable versions.