Best explanation?
Maybe? My take is, knowing what causes elongation should make you Not use a "in between the rollers" chain gauge. Least if elongation is what you want to gauge. If overall wear is what you are after its fine, but just be aware roller wear adds a lot to apparent elongation.
From the video it appears as if the chain is "extremely worn" (he says) measured with the park tool gauge, but the same chain measured with the blue ruler shows less than 1/16" elongation in 12". That is at worst 0.5%. That doesn't qualify as Extreme. Not by a long shot.
Of course we can argue all day if roller wear should be included. I think it should, but implemented in the gauge to account for both true elongation and apparent elongation, to not cause you to prematurely discard the chain.