This is the only important point. The question is 1% technology and 99% audience.
I think when we say "audience", it can mean different things.
For example, Medium has potential to bring a huge "instant" audience, but there's a difference between instant audience and long term audience.
I just went and took a look at all the large open source projects, and they all have their own blog on their own domain. AND, what comes next is important. They have:
1. RSS feed button
2. Link to subscribe to the blog via email
I think these are important in the long term if you are thinking about looking at your open source project from a long term point of view. Medium may give you instant traffic, but most of them don't really care about what you have to say.
Whereas having your own custom domain with a way for people to subscribe via RSS or email will give you a long term audience who are actually genuinely interested in your project. And with those formats it is very likely that your message will reach them, compared to medium where it may just flow away from their timeline without them ever noticing.
Neither an RSS feed nor a subscribe link enable people to find you in the first place. They do, however, make it more likely that they'll come back after finding you.
You should absolutely also build retention techniques into your Medium posts. And your custom blog. And anywhere else where you care about people reading your content, and keeping on reading it.
You could add a link to your RSS feed to the bottom of each post as a workaround, but this is far from ideal.
I agree that self-hosting + custom domain is better in this aspect.
[0] https://help.medium.com/hc/en-us/articles/214874118-RSS-Feed...