medium.com stories are penalized by default on HN, as are most sites that produce solid articles amid a lot of fluff. (If it were only fluff, we'd ban the site.) The penalty can get lifted by moderators or by software.

You may be underestimating the sheer quantity of articles people post from there (dozens a day), as well as the diversity of topics in them. I took a quick look and the 6 most successful medium.com posts from yesterday were all technical. (You can check this for yourself by clicking on the domain name next to the story title, which shows you all the posts from that domain.)