This is part where we disagree. Not that I am defending that private entities should "control" public speech, but rather that this control is circunstancial. Remove Facebook and Twitter (and every big media conglomerate as well, FOX, CNN, NBC) all you want, people will still look for groups that share their views and messages that confirm their biases.
This is not just a guess. I am seeing this first-hand with the people looking into leaving Twitter and joining Mastodon. Go to /r/mastodon and you will see me arguing with every one that comes with the idea that different instances mean different "communities" and "interests".
Also, consider the alternative. The article is saying that we shouldn't want a decentralized web. Who would you propose to "control" public speech? If not private companies and if not smaller groups, the only alternatives left is, guess what, Big State and tyrants