I'm not seeing anywhere on the AEF site that indicates that the source is released under the GPL. Just because it is free software, does not mean that it automatically falls under the GPL. It appears as though they created their own license, which does in fact prohibit you from re-branding and distributing the source and/or distro files. At best, AEF has a modified MSD or MIT license. Vanilla Forums and BuddyPress are the two most popular open source packages that use a GPL license.