Is it really free speech if you are whispering it behind closed doors? What happens when someone else views the same thread and wants to see an opposing viewpoint to mine? What if they are never exposed to the opposing viewpoint in their lives, ever? They can't read our private messages, and this builds a closed public discussion on a narrow set of viewpoints. That policy creates the public image of a discussion that is controlled to be exactly one viewpoint, and all the real discourse is in private. That is practically the definition of a sounding chamber. Lock your subversive thoughts up in private.
There is not and can not be a dichotomy of free speech or censorship. If someone wants to see more of what Grendel wants to say, they click on his name. There they can currently see him posting a video about feminists deserving to be whipped, and decide if they still want to see his idea of "reasonable disagreement". If they do, they can follow and/or mail him. OP muting him doesn't prevent this. A user who actually wants to hear from Grendel and is capable enough to find hubski isn't too stupid to figure this out.
How can you click on the name of someone you don't know exists? You only currently know about him because I told you about him in this context. If you visited the original thread, you would not have known he existed, and there would have been no name to click on. You seem to be blinded by the specific example of Grendel, let's take it to an abstract. When republicans visit a democratic convention or vice versa, imagine if they were just blocked from entering the building (which they are, usually you have to be a registered democrat/republican to go to those things). This creates a sounding chamber for all democrats to think democratly and all republicans to think republicanly. Now imagine if you didn't know the other party existed. Let's take the Mendelssohn party for example. It comes around, gets blocked at the door from all political debates, and never reaches public knowledge that the party even exists or what its ideals are. I, the average person, can't just go to the Mendelssohn party's homepage if I don't know that the Mendelssohn party exists.
Trust me when I say you didn't introduce me to Grendel. There are plenty of his thoughts hosted by Hubski. Your political party example confuses me. Political parties and politicians do not exist in a void. Mainstream media isn't in a void. People browsing the internet aren't in a void. People setting boundaries around their participation in the few places where there are tools to do so, is not going to create an echo chamber as dramatic as the one you are imagining. Nor is excluding someone from hijacking your platform.