There is a stanchion blocking off the F section (from the J side) and if you try to go in, the lounge desk people upstairs will ask for your boarding pass to check if you are eligible. So without a valid lounge invite or boarding pass, you can't enter. And the reason there is a stanchion blocking the way is because that's technically the back entrance as the main entrance is next to the immigration on the upper level (level 7).