Measure and read across a square or rectangle, diagonally " X " insuring that both corners read the same. Seems to work well, no math adjustment, correction needed. Check the corners with a square then try to assemble square.
If welding tack the corners diagonally, verify and weld.
Wood assembly, one will be surprised at the amount of variation found, example HD "L" braces, or brackets. NP bracket, bend to adjust, brace skew to suit.
If a tape has an error from one to another, makes no difference .Should a different tape from the one used for layout be used when checking square.
The square can be checked accurately X , sure the measured reading of the 2 tapes will vary. That does not matter as long as the tape used reads the same at both corners.
Never saw a full square laid out & read, in that video, did not have the sound running, no need. Not rocket science