You don't know which headphones they used for testing. And you also don't know if these guys figured out that you have to activate the HiFi DAC manually when you use the 3.5mm jack for the first time (after that, it's on by default). GSMarena's audio testing is not very proficient. Also worth mentioning: High Impedance mode is only activated above 75ohm, as far as I know.
Anyway: With my IEMs and Headphones, the G7 gets very loud.
Edit: Did some research. Apparently they're using 32ohm headphones. Of course that does not activate the high impedance mode. Moreover, If the DAC was activated, it would have run in Line-Out mode for normal testing and then switched to low-impedance-headphone mode, which is a huge huge difference in volume.