My first thought when I read the article was that that image must have been run through something like a contrast-limited adaptive histogram equalization (CLAHE

My first thought when I read the article was that that image must have been run through something like a contrast-limited adaptive histogram equalization (CLAHE) process. See here, for an example: https://imagemagick.org/script/clahe.php https://imagemagick.org/script/clahe.php