This argument might be more effective were it not stereotyping those who believe in free speech as a member of the alt-right. I am a progressive Democrat who has been targeted for disagreeing with political ideas of some large constituencies and consider their actions worse than anything I have heard about from the alt-right.
It seems to me social psychological theory accounts for most of the excesses on either side, and this is backed by a great deal of research into user experiences. In-group / out-group homogeneity does apply to online behavior, and groups tend to assert their positions with a level of aggression proportional to the difference in numbers. Political persuasion does not seem to be a factor in whether or not this occurs.
The other thing I wanted to point out is the article presupposes a set of fixed norms for online communication that are primarily centered on speech. Technology tends to change, and the importance of platforms like Twitter will likely diminish in the future. Decentralized peer-to-peer networking is a thing, and it’s likely that the pub-sub models that encourage brigading will be made obsolete before long in favor of more private communications channels.