Substack is an "absolute free speech" app. They have gotten away with it so far because nobody could see the highly offensive content unless they actively sought after it and subscribed.
However now that Substack is becoming more like a conventional social media platform they will have a harder time being an "absolute free speech" app.
You make it sound like a bad thing. Being proud of harboring hate speech puts one in a very specific category of people, one I wouldn't be proud to be part of.
One thing that is often lost with the free speech argument is amplification. For example, no one is stopping most people from going and yelling on a street corner. But that doesn't mean that you need to be allowed to speak in a particular venue or platform. Our society is currently not one in which there is a right to "equal volume" on speech, and I think that's a good thing.
The entire free speech movement was a response to government literally throwing people in jail for even talking about a given topic. It was never about allowing minority opinions to be as loud as majority opinions.
It's a good thing, I think. "Absolute free speech" is most often championed by people who want to build society around pure logic rather than empirical evidence. The slippery slope bogeyman they gesture to is laughably dated in a time when misinformation and misdirection are so much easier.
However now that Substack is becoming more like a conventional social media platform they will have a harder time being an "absolute free speech" app.