Those timestamps are likely from the image metadata, there's no reason to believe that the images are out of order from their numeric/shortcode ids.

What the author refers to as the "real post #1" has a taken_at_timestamp less than the first picture. It's still id #6 in the database versus #2 (the dog picture). Numeric ids are easily visible in the source of the page or by using this not-so-secret query string: https://www.instagram.com/p/C/?__a=1