Hmmmm... I'm confused by the text. What does, "Two consecutive tones are always separated by a single semitone," mean? Does it refer to to tones consisting of two pure pitches?
In listening to the example, that would not close, I heard the first sound as being lower than the second sound.
I think I am not getting something here.
BTW, I think the AA post is extremely well worded. I'm impressed with the author.