This was just a demo that was not finished during the class. The recommendation is to install the metal U-channel over that outside edge to reinforce it. Schluter tried one this size, with the metal, tile, and after properly curing, put 12 bags of thinset on it before it broke away. Prior to that, the neither tile nor grout cracked. With the metal U-channel, you can use it for a free-standing end wall and tile it without worries. Have you seen any of the Wedi free-standing shower kits? these things are stable. I think the foam in KerdiBoard is a little denser and the tileable layer adds stiffness, but have not measured it. If you're going to sit or stand on it, you need to reinforce it just like your plywood decking material, in the order of every 16" OC. Done this way, you have a quick, solid, easy means to create a tub deck, steps, vanities, pipe covers, and pretty much anything you can think of, and with the pre-scored stuff, easy curves as well. That picture was taken with my cellphone, and no guarantees on the optics quality. It probably did sag a little, but again, that is without fully curing the side attachment points, without the metal reinforcement channel, and no tile bonded, which all would add significantly to the overall strength and stiffness. It was a demo that was not finished, nor done with huge care, just to get the idea of what was possible. We would have made the shelf's edges perfect to the corner so it had a more perfect surface bond - actually easy, but we didn't take the time. Tried it, found out how to do it, and then went on to the next concept. None of those modules were anything permanent - they were all going to be torn apart, and the basic wall/floor reused for the next class.