I can confirm that V_BCKP (pin 6) is not connected (measured with multimeter) to VCC_3V3 (nor GPS_3V3, nor VBATT nor VBIN) .
One would expect it to be connected to VCC_3V3 (which is battery powered). I also do not see a solder jumper that could enable it.
With some soldering craftmanship V_BCKP may be manually connected to VCC_3V3. According to the datasheet that would draw 15 uA (@3.0V) backup current for the MAX-7Q.
The power for the GPS can be switched off completely (with PA15), so if correctly switched off the GPS should not contribute to the measured 1.5mA in ‘low power’ / sleep.