From the article, it looks like Gwern, at least, thought quite a lot about text justification --- see the "Abandoned" section --- so the "learn how typography works" comment seems a bit unfair.
My point being that applying rules learnt from hundreds of years of literary culture and printing do not 1:1 translate into digital.
Justified copy is as much about saving paper as it is about any formality/legibility. It really has no place in long form digital space where space is infinite.
Type designers like consistent and even rhythm between the ink and page. Rivers of white and large gaps between words give the ick.
