So the author wants to protect "the marketplace of ideas" from monopolies caused by a different market. However, they don't want the government to do it. They also don't seem to think it's possible for smaller companies to challenge these monopolies. What then is the author's solution? One is never given.
In my opinion, the solution is simple: If any “platform” censors beyond removing illegal or copyrighted content or spam, they should have their “platform” status revoked, and they should be considered a publisher. This prevents such companies from using “platform” status as a cost saving mechanism to help them become large monopolies. Companies like Facebook would not have been able to take over so much of the market…
edit: What about downvoting? Does that not count because it's by users? However the algorithm for ranking and matching is controlled by the platform, does that count as censorship since it will promote some posts over others?