xfce4-screensaver 0.1.8-2 The "Activate screensaver when computer is idle" is configured for 30 min, but it activates at around 5-6 min. $ xset q ... Screen Saver: prefer blanking: no allow exposures: no timeout: 1800 cycle: 1800 ... DPMS (Energy Star): Standby: 600 Suspend: 600 Off: 600 DPMS is Enabled Monitor is On It works correctly with external xscreensaver app. Lenovo Thinkpad R61i.
Correction: the screensaver activates at 10 min, which suggests DPMS trigger.
Indeed, it seems the screen lock activates when the screens blanks through other means. You can test this using `xset dpms force off` and then waiting 1 minute (or whatever your "Lock screen after screensaver is active for X minutes" is).
Likely a duplicate of #16327, the DPMS bug. *** This bug has been marked as a duplicate of bug 16327 ***