The location_mode of the Secure settings will disable the Location service until it will be turned on on the settings or by changing the value of the key location_mode. (Checks if location is enabled, based on mode and restrictions.)
Disabling the location mode imply that applications cannot get the fix to access the Location services. Third-party applications on Android cannot access the GPS/GNSS hardware, it can access the services of GNSS and Location but not the hardware itself. To access these services, the application need to have the permission ACCESS_FINE_LOCATION or ACCESS_COARSE_LOCATION AND the Location service must be enabled.
On other hand, system applications can have access to the GNSS/GPS hardware (permission LOCATION_HARDWARE). Thus, there is no guarantee that GNSS/GPS will not be used by the system itself.