This is true, but not for 2F/2G lounges.

You can access 2F lounge even if your flight departs from 2G. No need to ask anyone. As reported here, just scan your boarding pass and the gate will open.

The issue is, that it is far away from your 2G departure gate and you need to allow time (and buffer) to head to your gate by bus for boarding.

If you need a shower (and have enough time), if what the last poster says is right (why would he not tell the truth), go to 2F for your shower.

Then take the shuttle bus to 2G. The lounge is quite nice. Not as good as the 2F lounge but nice. And you can enjoy without stress till when boarding is announced.