You’re discounting the security/authority that “medium.com” brings over a random domain name. You might get less traffic from HN and Reddit if you weren’t on Medium.
Edit: just because a few people (myself included) have a negative reaction to a Medium link doesn’t change the fact that many Medium links are upvoted on HN.
At least for me and some other people, medium.com is a negative signal of quality. Medium is full of low-quality and marketing articles, I'll associate your links with that, and me only seeing "medium.com" on e.g. HN means you don't have a chance of establishing a "brand" I trust by recognizing your domain as "good" after a few articles.
EDIT: adressing your edit, do you have any evidence they wouldn't be if they weren't on medium.com?
I think it goes for any topic. I’ll use food as an example. I’ve been on a lot of food blogs, I end there from DuckDuckGo or google, mostly google.
I only remember a handful of them, one in particular that is now a regular bookmark of mine (it’s https://www.valdemarsro.dk and in Danish so likely uninteresting to most), and the one thing that the ones I remember have in common is that they are their own things.
I don’t think there is anything beneficial to being a medium blog for this exact reason. I may read your article, I may even really like it, but it takes more than one article to get me to stick around, and because every medium article looks the same I can easily end up reading more than one without noticing it.
Having a unique domain helps as much as having a unique look.
Don't forget that it also prominently features a large distracting photo that was just ripped from a royalty-free photo site and has nothing to do with the content.
It worked. Then someone wrote about it, and everyone did it. Then it became a negative signal. There seems to be a 5-10 year trend lag time on stuff like that where the least self-aware self-promoters don't realize it doesn't work. Eventually a new trend falls to their level, and the cycle begins anew.
A big reason everyone has the big, irrelevant image on top is that medium was designed to work best with that assumption. If you actually blog there, your index won't look good unless you have a header photo for each post.
Edit: just because a few people (myself included) have a negative reaction to a Medium link doesn’t change the fact that many Medium links are upvoted on HN.