Earlier quoted context omitted.
The page you linked seems to do a reasonable job of answering that: "Using on your phone’s built-in GPS, Wi-Fi, and gyroscope to determine distances and the orientation of walls, RoomScan can reliably estimate measurements to within about half a foot, which should be adequate if all you need is a rough floor plan. If, you need more precise measurements, you can opt for the $5 Pro version, which allows you to specify…
I was suggesting GPS wouldn't work with most rooms having ceilings. Wifi positioning I would imagine is highly inaccurate, which leaves the gyroscope. Judging by some screenshots and appstore comments I assume its only roughly accurate, and requires you to move your phone slowly between walls in straight lines (not via your pocket or around furniture for example). I think I would have immediately dismissed an idea li…
The site claims it's accurate to within 6 inches which, based on a quick trial, seems about right.
You don't have to move it in a straight line as far as I can tell. I just moved it around the room and pressed it against the walls.
Like you, I wouldn't have expected this to work. But it pretty much does. I didn't experiment with what radio signals it needs to work. (Though for most uses I'm not sure it's easier than a laser distance meter and some graph paper.)