Not really. He's finding the difference between the axle height and the bb height.
BB height - axle height = BB rise.
His way of doing it was much simpler than getting a piece of string and holding it tight from axle to axle, and then holding a tape measure/ruler above the string which would be slightly slack anyway... Provided he did it on flat ground and measured perpendicular to the ground, it would be fine.
(Unless he had vastly different wheels - Like a 26" on the front and a 20" on the back :D )