I got a free one of those boxes from somewhere. It does look like a pretty good product. That ground screw goes into the bracket and that also gets you into the fan mounting screws which go all the way through. (the nuts in the picture)
It is a sturdy box.
I used to do a lot of Fans back in the day, and used the expandable fan braces quite a bit. The installations that gave me the most trouble were where I was replacing a ceiling box that was mounted to the side of a joist. Sometimes you couldn't get the fan box close enough where a small Fan canopy would totally cover the original hole.
These boxes, being flat on one side, would have done the trick, and saved me a lot of greif.
Bill