I agree with this, BUT.
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.