What firmware did you debrick to? That feature only came about in the last 3 or so firmware releases, so try upgrading your firmware.
Also is it possible that after you made changes in GPARTED that you didn't click the APPLY button? Without clicking that button, any changes you make to the drive don't actually get processed.
Also any mounted partitions can cause issues in GPARTED... I will change the guide to state you should wipe the drive clean from a Windows or MAC machine if you are gonna use the destroy flag.