I don't know what the policy of Hacker News here, but this article is NOT attributed to its author and is "sourced" from the essay the author had published on Medium a few days prior. To me this suggests that the article might not have been reproduced here either by or with the permission of the author.

To be fair, I don't assume that there's any especial reason that Medium rather than 'Write As' deserves the traffic that the article might generate, but on the other hand as a way to bump traffic to your own website, republishing a popular article without permission seems a little sketchy.