I think as soon the fstab is added, then it will use the compression. But the compression on f2fs is quite differently to the one from btrfs/zfs. It won’t free up storage.
So far I know, to check if compression is enabled you should be able to see this with:
sudo fsck.f2fs -fv /dev/sdX
Maybe also if you check in your dmesg log, it could show something:
sudo dmesg | grep f2fs
sudo dmesg | grep compress
Im not deeply into f2fs, sorry :/