The image in that link isn't correct.
As noted, they provide a small "shelf" on the back of the unit to mount the power supply to, any yes it does make a nice, clean install for an HA pair.
Other suggestions - Cisco use to have a "mounting" magnet they used on ths small PIX devices, almost like a mouse pad in size, then just "mount" it to a switch or anything else that has a metal chassis 🙂