Hmmm. These seem to be two different things here. The youtube video is how I have been taught to square up a quilt, large or small. To actually get it square, you align the ruler with a seam at the corner, usually the border but the seam of a block would work too. Measure, tug, measure and trim. It seems to me that the picture tutorial is simply trimming away the batting and backing. I'm not sure that actually squares it up.
But then, I could be wrong.