It's probably just to do with the frame being so small and there being no room for a hanger on the frame, so it uses the BMX style system with the separate hanger.
Pretty sure it's real, seem to remember this set up being raised on a Zaskar thread a while ago, it is under the seatstay due lack of space on the top, was only like this on the 14.5 frame.
Gentleman the frame in the listing is a 1993 14.5". As most of you remember when Shimano switched to a lower profile cantilever design in 1993 GT switched most of the line to cantilevers at that time. (Main reason GT stayed with the u-brakes as long as they did is because heels would clip the brakes until their design became lower profile.) However the 14.5" frame still had such a low seat stay that they chose to stick with the u-brake on the 14.5" frame. They chose the under stay position because it was cleaner. My daughter has a 93 in purple ano with the brake under the seat stays