They could use digital photographs and photogrammetry software, but it looks to me like they are probably using structured light depth cameras in this case.
I would be fairly confident in betting that it works well if you are losing or gaining weight without much change in muscle mass. If you are a more advanced lifter or are gaining a lot of muscle mass, I doubt it gets measured anywhere close to accurately.
It should be a very accurate way to visually assess changes in your body composition over time if you are looking at the scan itself (not their numbers). A few of the measurements, like the waist circumference could also be of value with this method.
Do they give you a copy of the scan or is it all locked into their software? If you get a copy, you could do some interesting analysis on your own.
Do you know what file type the 3D renders are given as? Probably doesn't make a difference to most people, but I am curious.