xscreensaver seems to be partially to blame. After uninstalling it, logging out and logging in, xset q reports
Screen Saver:
prefer blanking: yes allow exposures: yes
timeout: 240 cycle: 600
...
DPMS (Energy Star):
Standby: 600 Suspend: 0 Off: 900
DPMS is Enabled
Monitor is On
So the 4 minute blanking timeout is now respected, but the DPMS timeout has been set to 10 minutes instead of the requested 6 (and the DPMS off time has been set to 15 minutes, instead of 0 as in the xset command written by light-locker-settings).
xscreensaver seems to be partially to blame. After uninstalling it, logging out and logging in, xset q reports
Screen Saver:
prefer blanking: yes allow exposures: yes
timeout: 240 cycle: 600
...
DPMS (Energy Star):
Standby: 600 Suspend: 0 Off: 900
DPMS is Enabled
Monitor is On
So the 4 minute blanking timeout is now respected, but the DPMS timeout has been set to 10 minutes instead of the requested 6 (and the DPMS off time has been set to 15 minutes, instead of 0 as in the xset command written by light-locker- settings) .